diff --git a/src/renderer/components/layout/PaneContent.test.tsx b/src/renderer/components/layout/PaneContent.test.tsx
new file mode 100644
index 00000000..57ce64a1
--- /dev/null
+++ b/src/renderer/components/layout/PaneContent.test.tsx
@@ -0,0 +1,136 @@
+/* eslint-disable @typescript-eslint/naming-convention -- Component mocks mirror PascalCase exports. */
+import React, { act } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import type { Pane } from '@renderer/types/panes';
+import type { Tab } from '@renderer/types/tabs';
+
+vi.mock('../dashboard/DashboardView', () => ({
+  DashboardView: () => React.createElement('div', { 'data-view': 'dashboard' }, 'Dashboard view'),
+}));
+
+vi.mock('../extensions/ExtensionStoreView', () => ({
+  ExtensionStoreView: () =>
+    React.createElement('div', { 'data-view': 'extensions' }, 'Extension store view'),
+}));
+
+/* eslint-enable @typescript-eslint/naming-convention -- Re-enable after component mocks. */
+
+import { PaneContent } from './PaneContent';
+
+const flushReact = async (): Promise<void> => {
+  await Promise.resolve();
+  await Promise.resolve();
+  await new Promise<void>((resolve) => {
+    setTimeout(resolve, 0);
+  });
+};
+
+const roots: Root[] = [];
+
+const dashboardTab: Tab = {
+  id: 'tab-dashboard',
+  type: 'dashboard',
+  label: 'Dashboard',
+  createdAt: 1,
+};
+
+const extensionTab: Tab = {
+  id: 'tab-extensions',
+  type: 'extensions',
+  label: 'Extensions',
+  createdAt: 2,
+};
+
+const createPane = (tabs: Tab[], activeTabId: string | null): Pane => ({
+  id: 'pane-main',
+  tabs,
+  activeTabId,
+  selectedTabIds: [],
+  widthFraction: 1,
+});
+
+const createHarness = (): { host: HTMLDivElement; root: Root } => {
+  const host = document.createElement('div');
+  document.body.appendChild(host);
+  const root = createRoot(host);
+  roots.push(root);
+  return { host, root };
+};
+
+const renderPane = async (root: Root, pane: Pane): Promise<void> => {
+  await act(async () => {
+    root.render(<PaneContent pane={pane} isPaneFocused />);
+    await flushReact();
+  });
+};
+
+const waitForText = async (host: HTMLElement, text: string): Promise<void> => {
+  for (let attempt = 0; attempt < 10; attempt += 1) {
+    if (host.textContent?.includes(text)) {
+      return;
+    }
+
+    await act(async () => {
+      await flushReact();
+    });
+  }
+
+  expect(host.textContent).toContain(text);
+};
+
+describe('PaneContent', () => {
+  beforeEach(() => {
+    vi.stubGlobal('IS_REACT_ACT_ENVIRONMENT', true);
+  });
+
+  afterEach(async () => {
+    await act(async () => {
+      for (const root of roots.splice(0)) {
+        root.unmount();
+      }
+      await flushReact();
+    });
+    document.body.innerHTML = '';
+    vi.unstubAllGlobals();
+  });
+
+  it('renders the default dashboard without suspending when no tabs are open', async () => {
+    const { host, root } = createHarness();
+
+    await renderPane(root, createPane([], null));
+
+    expect(host.textContent).toContain('Dashboard view');
+    expect(host.querySelector('[role="status"]')).toBeNull();
+  });
+
+  it('does not mount inactive lazy tab content during initial pane render', async () => {
+    const { host, root } = createHarness();
+
+    await renderPane(root, createPane([dashboardTab, extensionTab], dashboardTab.id));
+
+    expect(host.textContent).toContain('Dashboard view');
+    expect(host.textContent).not.toContain('Extension store view');
+    expect(host.querySelector('[role="status"]')).toBeNull();
+  });
+
+  it('loads a lazy tab on first activation and keeps it mounted after switching away', async () => {
+    const { host, root } = createHarness();
+
+    await renderPane(root, createPane([dashboardTab, extensionTab], dashboardTab.id));
+    expect(host.textContent).not.toContain('Extension store view');
+
+    await renderPane(root, createPane([dashboardTab, extensionTab], extensionTab.id));
+    await waitForText(host, 'Extension store view');
+
+    const extensionView = host.querySelector<HTMLElement>('[data-view="extensions"]');
+    expect(extensionView).not.toBeNull();
+
+    await renderPane(root, createPane([dashboardTab, extensionTab], dashboardTab.id));
+
+    expect(host.querySelector('[data-view="extensions"]')).toBe(extensionView);
+    expect(extensionView?.closest<HTMLElement>('.absolute')?.style.display).toBe('none');
+  });
+});
diff --git a/src/renderer/components/layout/PaneContent.tsx b/src/renderer/components/layout/PaneContent.tsx
index b4d96187..d7d71e43 100644
--- a/src/renderer/components/layout/PaneContent.tsx
+++ b/src/renderer/components/layout/PaneContent.tsx
@@ -3,27 +3,135 @@
  * Uses CSS display-toggle to keep all tabs mounted (preserving state).
  */
 
-import { TeamGraphTab } from '@features/agent-graph/renderer';
+import { lazy, Suspense, useEffect, useState } from 'react';
+
 import { TabUIProvider } from '@renderer/contexts/TabUIContext';
 
 import { DashboardView } from '../dashboard/DashboardView';
-import { ExtensionStoreView } from '../extensions/ExtensionStoreView';
-import { NotificationsView } from '../notifications/NotificationsView';
-import { SessionReportTab } from '../report/SessionReportTab';
-import { SchedulesView } from '../schedules/SchedulesView';
-import { SettingsView } from '../settings/SettingsView';
-import { TeamDetailView } from '../team/TeamDetailView';
-import { TeamListView } from '../team/TeamListView';
-
-import { SessionTabContent } from './SessionTabContent';
 
 import type { Pane } from '@renderer/types/panes';
+import type { Tab } from '@renderer/types/tabs';
+
+const ExtensionStoreView = lazy(() =>
+  import('../extensions/ExtensionStoreView').then((module) => ({
+    default: module.ExtensionStoreView,
+  }))
+);
+const NotificationsView = lazy(() =>
+  import('../notifications/NotificationsView').then((module) => ({
+    default: module.NotificationsView,
+  }))
+);
+const SessionReportTab = lazy(() =>
+  import('../report/SessionReportTab').then((module) => ({
+    default: module.SessionReportTab,
+  }))
+);
+const SchedulesView = lazy(() =>
+  import('../schedules/SchedulesView').then((module) => ({
+    default: module.SchedulesView,
+  }))
+);
+const SettingsView = lazy(() =>
+  import('../settings/SettingsView').then((module) => ({
+    default: module.SettingsView,
+  }))
+);
+const TeamDetailView = lazy(() =>
+  import('../team/TeamDetailView').then((module) => ({
+    default: module.TeamDetailView,
+  }))
+);
+const TeamListView = lazy(() =>
+  import('../team/TeamListView').then((module) => ({
+    default: module.TeamListView,
+  }))
+);
+const SessionTabContent = lazy(() =>
+  import('./SessionTabContent').then((module) => ({
+    default: module.SessionTabContent,
+  }))
+);
+const TeamGraphTab = lazy(() =>
+  import('@features/agent-graph/renderer').then((module) => ({
+    default: module.TeamGraphTab,
+  }))
+);
 
 interface PaneContentProps {
   pane: Pane;
   isPaneFocused: boolean;
 }
 
+interface PaneTabSlotProps {
+  tab: Tab;
+  isActive: boolean;
+  isPaneFocused: boolean;
+}
+
+const PaneLazyFallback = (): React.JSX.Element => (
+  <div className="flex flex-1 items-center justify-center bg-surface">
+    <div
+      className="size-5 animate-spin rounded-full border border-border border-t-text-muted"
+      aria-label="Loading tab"
+      role="status"
+    />
+  </div>
+);
+
+const PaneTabSlot = ({ tab, isActive, isPaneFocused }: PaneTabSlotProps): React.JSX.Element => {
+  const [hasActivated, setHasActivated] = useState(isActive);
+
+  useEffect(() => {
+    if (isActive) {
+      setHasActivated(true);
+    }
+  }, [isActive]);
+
+  return (
+    <div className="absolute inset-0 flex" style={{ display: isActive ? 'flex' : 'none' }}>
+      {hasActivated && (
+        <Suspense fallback={<PaneLazyFallback />}>
+          {tab.type === 'dashboard' && <DashboardView />}
+          {tab.type === 'notifications' && <NotificationsView />}
+          {tab.type === 'settings' && <SettingsView />}
+          {tab.type === 'teams' && <TeamListView />}
+          {tab.type === 'team' && (
+            <TabUIProvider tabId={tab.id}>
+              <TeamDetailView
+                teamName={tab.teamName ?? ''}
+                isActive={isActive}
+                isPaneFocused={isPaneFocused}
+              />
+            </TabUIProvider>
+          )}
+          {tab.type === 'session' && (
+            <TabUIProvider tabId={tab.id}>
+              <SessionTabContent tab={tab} isActive={isActive} />
+            </TabUIProvider>
+          )}
+          {tab.type === 'report' && <SessionReportTab tab={tab} />}
+          {tab.type === 'extensions' && (
+            <TabUIProvider tabId={tab.id}>
+              <ExtensionStoreView />
+            </TabUIProvider>
+          )}
+          {tab.type === 'schedules' && <SchedulesView />}
+          {tab.type === 'graph' && (
+            <TabUIProvider tabId={tab.id}>
+              <TeamGraphTab
+                teamName={tab.teamName ?? ''}
+                isActive={isActive}
+                isPaneFocused={isPaneFocused}
+              />
+            </TabUIProvider>
+          )}
+        </Suspense>
+      )}
+    </div>
+  );
+};
+
 export const PaneContent = ({ pane, isPaneFocused }: PaneContentProps): React.JSX.Element => {
   const activeTabId = pane.activeTabId;
 
@@ -41,46 +149,7 @@ export const PaneContent = ({ pane, isPaneFocused }: PaneContentProps): React.JS
       {pane.tabs.map((tab) => {
         const isActive = tab.id === activeTabId;
         return (
-          <div
-            key={tab.id}
-            className="absolute inset-0 flex"
-            style={{ display: isActive ? 'flex' : 'none' }}
-          >
-            {tab.type === 'dashboard' && <DashboardView />}
-            {tab.type === 'notifications' && <NotificationsView />}
-            {tab.type === 'settings' && <SettingsView />}
-            {tab.type === 'teams' && <TeamListView />}
-            {tab.type === 'team' && (
-              <TabUIProvider tabId={tab.id}>
-                <TeamDetailView
-                  teamName={tab.teamName ?? ''}
-                  isActive={isActive}
-                  isPaneFocused={isPaneFocused}
-                />
-              </TabUIProvider>
-            )}
-            {tab.type === 'session' && (
-              <TabUIProvider tabId={tab.id}>
-                <SessionTabContent tab={tab} isActive={isActive} />
-              </TabUIProvider>
-            )}
-            {tab.type === 'report' && <SessionReportTab tab={tab} />}
-            {tab.type === 'extensions' && (
-              <TabUIProvider tabId={tab.id}>
-                <ExtensionStoreView />
-              </TabUIProvider>
-            )}
-            {tab.type === 'schedules' && <SchedulesView />}
-            {tab.type === 'graph' && (
-              <TabUIProvider tabId={tab.id}>
-                <TeamGraphTab
-                  teamName={tab.teamName ?? ''}
-                  isActive={isActive}
-                  isPaneFocused={isPaneFocused}
-                />
-              </TabUIProvider>
-            )}
-          </div>
+          <PaneTabSlot key={tab.id} tab={tab} isActive={isActive} isPaneFocused={isPaneFocused} />
         );
       })}
     </div>
diff --git a/src/renderer/components/layout/Sidebar.test.tsx b/src/renderer/components/layout/Sidebar.test.tsx
new file mode 100644
index 00000000..43a34441
--- /dev/null
+++ b/src/renderer/components/layout/Sidebar.test.tsx
@@ -0,0 +1,111 @@
+/* eslint-disable @typescript-eslint/naming-convention -- Component mocks mirror PascalCase exports. */
+import React, { act } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+const storeMock = vi.hoisted(() => ({
+  state: {
+    sidebarCollapsed: false,
+    toggleSidebar: vi.fn(),
+  },
+}));
+
+vi.mock('@renderer/store', () => ({
+  useStore: <T,>(selector: (state: typeof storeMock.state) => T): T => selector(storeMock.state),
+}));
+
+vi.mock('../sidebar/GlobalTaskList', () => ({
+  GlobalTaskList: () => React.createElement('div', { 'data-testid': 'tasks-panel' }, 'Tasks panel'),
+}));
+
+vi.mock('../sidebar/DateGroupedSessions', () => ({
+  DateGroupedSessions: () =>
+    React.createElement('div', { 'data-testid': 'sessions-panel' }, 'Sessions panel'),
+}));
+
+/* eslint-enable @typescript-eslint/naming-convention -- Re-enable after component mocks. */
+
+import { Sidebar } from './Sidebar';
+
+const roots: Root[] = [];
+
+const flushReact = async (): Promise<void> => {
+  await Promise.resolve();
+  await Promise.resolve();
+};
+
+const createHarness = (): { host: HTMLDivElement; root: Root } => {
+  const host = document.createElement('div');
+  document.body.appendChild(host);
+  const root = createRoot(host);
+  roots.push(root);
+  return { host, root };
+};
+
+const renderSidebar = async (root: Root): Promise<void> => {
+  await act(async () => {
+    root.render(<Sidebar />);
+    await flushReact();
+  });
+};
+
+function findButtonByText(host: HTMLElement, text: string): HTMLButtonElement {
+  const button = Array.from(host.querySelectorAll('button')).find(
+    (candidate) => candidate.textContent?.trim() === text
+  );
+  if (!(button instanceof HTMLButtonElement)) {
+    throw new Error(`Button not found: ${text}`);
+  }
+  return button;
+}
+
+describe('Sidebar', () => {
+  beforeEach(() => {
+    vi.stubGlobal('IS_REACT_ACT_ENVIRONMENT', true);
+    storeMock.state.sidebarCollapsed = false;
+    storeMock.state.toggleSidebar.mockClear();
+  });
+
+  afterEach(async () => {
+    await act(async () => {
+      for (const root of roots.splice(0)) {
+        root.unmount();
+      }
+      await flushReact();
+    });
+    document.body.innerHTML = '';
+    vi.unstubAllGlobals();
+  });
+
+  it('does not mount the sessions panel before the sessions tab is opened', async () => {
+    const { host, root } = createHarness();
+
+    await renderSidebar(root);
+
+    expect(host.querySelector('[data-testid="tasks-panel"]')).not.toBeNull();
+    expect(host.querySelector('[data-testid="sessions-panel"]')).toBeNull();
+  });
+
+  it('mounts the sessions panel on first activation and keeps it mounted when hidden', async () => {
+    const { host, root } = createHarness();
+    await renderSidebar(root);
+
+    await act(async () => {
+      findButtonByText(host, 'Sessions').click();
+      await flushReact();
+    });
+
+    const sessionsPanel = host.querySelector('[data-testid="sessions-panel"]');
+    expect(sessionsPanel).not.toBeNull();
+    expect(sessionsPanel?.closest<HTMLElement>('[role="tabpanel"]')?.hidden).toBe(false);
+
+    await act(async () => {
+      findButtonByText(host, 'Tasks').click();
+      await flushReact();
+    });
+
+    expect(host.querySelector('[data-testid="sessions-panel"]')).toBe(sessionsPanel);
+    expect(sessionsPanel?.closest<HTMLElement>('[role="tabpanel"]')?.hidden).toBe(true);
+  });
+});
diff --git a/src/renderer/components/layout/Sidebar.tsx b/src/renderer/components/layout/Sidebar.tsx
index b6f07b7a..8568a1b5 100644
--- a/src/renderer/components/layout/Sidebar.tsx
+++ b/src/renderer/components/layout/Sidebar.tsx
@@ -37,6 +37,7 @@ export const Sidebar = (): React.JSX.Element => {
   const [width, setWidth] = useState(DEFAULT_WIDTH);
   const [isResizing, setIsResizing] = useState(false);
   const [sidebarTab, setSidebarTab] = useState<SidebarTab>('tasks');
+  const [hasOpenedSessionsTab, setHasOpenedSessionsTab] = useState(false);
   const [taskFilters, setTaskFilters] = useState<TaskFiltersState>(defaultTaskFiltersState);
   const [taskFiltersPopoverOpen, setTaskFiltersPopoverOpen] = useState(false);
   const [isCollapseHovered, setIsCollapseHovered] = useState(false);
@@ -77,6 +78,12 @@ export const Sidebar = (): React.JSX.Element => {
     };
   }, [isResizing, handleMouseMove, handleMouseUp]);
 
+  useEffect(() => {
+    if (sidebarTab === 'sessions') {
+      setHasOpenedSessionsTab(true);
+    }
+  }, [sidebarTab]);
+
   const handleResizeStart = (e: React.MouseEvent): void => {
     e.preventDefault();
     setIsResizing(true);
@@ -195,7 +202,7 @@ export const Sidebar = (): React.JSX.Element => {
           hidden={sidebarTab !== 'sessions'}
           className="min-w-0 flex-1 overflow-hidden"
         >
-          <DateGroupedSessions />
+          {hasOpenedSessionsTab && <DateGroupedSessions />}
         </div>
       </div>
 
diff --git a/src/renderer/store/index.ts b/src/renderer/store/index.ts
index d0d655d1..3d651676 100644
--- a/src/renderer/store/index.ts
+++ b/src/renderer/store/index.ts
@@ -215,11 +215,10 @@ export function initializeNotificationListeners(): () => void {
   cleanupFns.push(() => {
     useStore.getState().unsubscribeProvisioningProgress();
   });
-  // Initial data fetches. Config loads first (needed for theme), then the rest
-  // run in parallel (no data dependencies between them). UV_THREADPOOL_SIZE=16
-  // prevents thread pool saturation even with concurrent I/O on Windows.
-  // Components also fire these from useEffect — loading guards in each action
-  // prevent duplicate IPC calls (whichever caller starts first wins).
+  // Initial data fetches. Config loads first (needed for theme), then the
+  // immediately visible data follows. Repository grouping is owned by the
+  // Sessions sidebar tab when it first mounts; scanning it here made startup
+  // pay for a hidden panel.
   void (async () => {
     // Config: fast (in-memory read) — needed for theme before first paint.
     await useStore.getState().fetchConfig();
@@ -262,10 +261,8 @@ export function initializeNotificationListeners(): () => void {
       runtimeStatusTimer = null;
     }, STARTUP_RUNTIME_STATUS_IDLE_DELAY_MS);
 
-    // Remaining fetches have no data dependency on each other — run in parallel
-    // to avoid blocking teams/notifications behind a slow repository scan.
+    // Remaining visible startup fetches have no data dependency on each other.
     await Promise.all([
-      useStore.getState().fetchRepositoryGroups(),
       useStore.getState().fetchAllTasks(),
       useStore.getState().fetchTeams(),
       useStore.getState().fetchNotifications(),
diff --git a/test/renderer/store/teamChangeThrottle.test.ts b/test/renderer/store/teamChangeThrottle.test.ts
index 162d2270..4ce938e3 100644
--- a/test/renderer/store/teamChangeThrottle.test.ts
+++ b/test/renderer/store/teamChangeThrottle.test.ts
@@ -82,6 +82,8 @@ vi.mock('@renderer/api', () => ({
   },
 }));
 
+import { api } from '@renderer/api';
+
 import { initializeNotificationListeners, useStore } from '../../../src/renderer/store';
 import { __resetTeamSliceModuleStateForTests } from '../../../src/renderer/store/slices/teamSlice';
 import {
@@ -90,7 +92,6 @@ import {
   summarizeTeamRefreshFanout,
   type TeamRefreshFanoutSnapshot,
 } from '../../../src/renderer/store/teamRefreshFanoutDiagnostics';
-import { api } from '@renderer/api';
 
 describe('team change throttling', () => {
   let cleanup: (() => void) | null = null;
@@ -190,6 +191,17 @@ describe('team change throttling', () => {
     expect(fetchTeamsSpy).toHaveBeenCalledTimes(1);
   });
 
+  it('does not scan repository groups during centralized startup initialization', async () => {
+    const getRepositoryGroupsSpy = vi.mocked(api.getRepositoryGroups);
+    getRepositoryGroupsSpy.mockClear();
+
+    cleanup?.();
+    cleanup = initializeNotificationListeners();
+    await vi.advanceTimersByTimeAsync(0);
+
+    expect(getRepositoryGroupsSpy).not.toHaveBeenCalled();
+  });
+
   it('allows next refresh after throttle window passes', async () => {
     const state = useStore.getState();
     const refreshTeamDataSpy = vi.spyOn(state, 'refreshTeamData');