diff --git a/src/renderer/components/team/activity/ActivityTimeline.tsx b/src/renderer/components/team/activity/ActivityTimeline.tsx
index d803c2de..abc3313d 100644
--- a/src/renderer/components/team/activity/ActivityTimeline.tsx
+++ b/src/renderer/components/team/activity/ActivityTimeline.tsx
@@ -75,6 +75,13 @@ const EMPTY_TEAM_NAMES: string[] = [];
 const EMPTY_TEAM_COLOR_MAP = new Map<string, string>();
 const DEFAULT_COLLAPSE_MODE = 'default' as const;
 
+function getItemSessionAnchorId(item: TimelineItem): string | undefined {
+  if (item.type === 'lead-thoughts') {
+    return item.group.thoughts[0]?.leadSessionId;
+  }
+  return undefined;
+}
+
 interface ItemCollapseProps {
   collapseMode: 'default' | 'managed';
   isCollapsed: boolean;
@@ -418,13 +425,20 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
     setVisibleCount(Infinity);
   };
 
-  const getItemSessionAnchorId = (item: TimelineItem): string | undefined => {
-    if (item.type === 'lead-thoughts') {
-      return item.group.thoughts[0]?.leadSessionId;
+  // Precompute, per timeline index, the most recent session anchor that appears
+  // strictly earlier in the list. Replaces an O(n) backward scan during render
+  // with an O(1) lookup; total work drops from O(n^2) to O(n) per timelineItems
+  // change.
+  const previousSessionAnchorByIndex = useMemo<readonly (string | undefined)[]>(() => {
+    const anchors: (string | undefined)[] = [];
+    let lastSeen: string | undefined;
+    for (const item of timelineItems) {
+      anchors.push(lastSeen);
+      const anchor = getItemSessionAnchorId(item);
+      if (anchor) lastSeen = anchor;
     }
-
-    return undefined;
-  };
+    return anchors;
+  }, [timelineItems]);
 
   // Pin the newest thought group (if first) so it stays at the top and doesn't jump.
   const pinnedThoughtGroup = timelineItems[0]?.type === 'lead-thoughts' ? timelineItems[0] : null;
@@ -532,14 +546,7 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
         let sessionSeparator: React.JSX.Element | null = null;
         if (realIndex > 0) {
           const currSessionId = getItemSessionAnchorId(item);
-          let prevSessionId: string | undefined;
-          for (let searchIndex = realIndex - 1; searchIndex >= 0; searchIndex -= 1) {
-            const candidateSessionId = getItemSessionAnchorId(timelineItems[searchIndex]);
-            if (candidateSessionId) {
-              prevSessionId = candidateSessionId;
-              break;
-            }
-          }
+          const prevSessionId = previousSessionAnchorByIndex[realIndex];
           if (prevSessionId && currSessionId && prevSessionId !== currSessionId) {
             sessionSeparator = (
               <div
diff --git a/test/renderer/components/team/activity/ActivityTimeline.test.ts b/test/renderer/components/team/activity/ActivityTimeline.test.ts
index fac0ca87..d02b5bb1 100644
--- a/test/renderer/components/team/activity/ActivityTimeline.test.ts
+++ b/test/renderer/components/team/activity/ActivityTimeline.test.ts
@@ -153,4 +153,135 @@ describe('ActivityTimeline session separators', () => {
       root.unmount();
     });
   });
+
+  it('renders a separator for every session transition across three lead sessions', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'thought-s3',
+        text: 'thought session 3',
+        leadSessionId: 'lead-session-3',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+      makeMessage({
+        messageId: 'thought-s2',
+        text: 'thought session 2',
+        leadSessionId: 'lead-session-2',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+      makeMessage({
+        messageId: 'thought-s1',
+        text: 'thought session 1',
+        leadSessionId: 'lead-session-1',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(React.createElement(ActivityTimeline, { messages, teamName: 'demo-team' }));
+    });
+
+    const matches = container.textContent?.match(/New session/g) ?? [];
+    expect(matches.length).toBe(2);
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it('finds the previous anchor even when many non-anchor items sit between lead thought groups', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'thought-newest',
+        text: 'newest thought',
+        leadSessionId: 'lead-session-newest',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+      ...Array.from({ length: 8 }, (_, i) =>
+        makeMessage({
+          messageId: `filler-${i}`,
+          text: `filler message ${i}`,
+          leadSessionId: `member-session-${i}`,
+          from: 'alice',
+          source: 'inbox',
+        })
+      ),
+      makeMessage({
+        messageId: 'thought-oldest',
+        text: 'oldest thought',
+        leadSessionId: 'lead-session-oldest',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(React.createElement(ActivityTimeline, { messages, teamName: 'demo-team' }));
+    });
+
+    expect(container.textContent).toContain('New session');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it('does not render a separator when two consecutive lead thoughts share the same session', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'thought-a',
+        text: 'thought a',
+        leadSessionId: 'lead-session-shared',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+      makeMessage({
+        messageId: 'thought-b',
+        text: 'thought b',
+        leadSessionId: 'lead-session-shared',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(React.createElement(ActivityTimeline, { messages, teamName: 'demo-team' }));
+    });
+
+    expect(container.textContent).not.toContain('New session');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it('handles a single message list without errors or separators', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'only',
+        text: 'only message',
+        leadSessionId: 'lead-session-1',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(React.createElement(ActivityTimeline, { messages, teamName: 'demo-team' }));
+    });
+
+    expect(container.textContent).not.toContain('New session');
+    expect(container.textContent).toContain('only message');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
 });