diff --git a/apps/web/src/components/review/DiffViewer.tsx b/apps/web/src/components/review/DiffViewer.tsx
index 5b9c1e2..d4cfd1e 100644
--- a/apps/web/src/components/review/DiffViewer.tsx
+++ b/apps/web/src/components/review/DiffViewer.tsx
@@ -1,9 +1,20 @@
 import type { FileDiff, DiffLine, ReviewComment } from "./types";
 import { FileCard } from "./FileCard";
 
+function getFileCommentMap(
+  commentsByLine: Map<string, ReviewComment[]>,
+  filePath: string,
+): Map<string, ReviewComment[]> {
+  const result = new Map<string, ReviewComment[]>();
+  for (const [key, val] of commentsByLine) {
+    if (key.startsWith(`${filePath}:`)) result.set(key, val);
+  }
+  return result;
+}
+
 interface DiffViewerProps {
   files: FileDiff[];
-  comments: ReviewComment[];
+  commentsByLine: Map<string, ReviewComment[]>;
   onAddComment: (
     filePath: string,
     lineNumber: number,
@@ -21,7 +32,7 @@ interface DiffViewerProps {
 
 export function DiffViewer({
   files,
-  comments,
+  commentsByLine,
   onAddComment,
   onResolveComment,
   onUnresolveComment,
@@ -37,7 +48,7 @@ export function DiffViewer({
         <div key={file.newPath} ref={(el) => onRegisterRef?.(file.newPath, el)}>
           <FileCard
             file={file}
-            comments={comments.filter((c) => c.filePath === file.newPath)}
+            commentsByLine={getFileCommentMap(commentsByLine, file.newPath)}
             onAddComment={onAddComment}
             onResolveComment={onResolveComment}
             onUnresolveComment={onUnresolveComment}
diff --git a/apps/web/src/components/review/FileCard.tsx b/apps/web/src/components/review/FileCard.tsx
index d7056c8..4d6ed6f 100644
--- a/apps/web/src/components/review/FileCard.tsx
+++ b/apps/web/src/components/review/FileCard.tsx
@@ -43,7 +43,7 @@ const leftBorderClass: Record<FileChangeType, string> = {
 
 interface FileCardProps {
   file: FileDiff;
-  comments: ReviewComment[];
+  commentsByLine: Map<string, ReviewComment[]>;
   onAddComment: (
     filePath: string,
     lineNumber: number,
@@ -60,7 +60,7 @@ interface FileCardProps {
 
 export function FileCard({
   file,
-  comments,
+  commentsByLine,
   onAddComment,
   onResolveComment,
   onUnresolveComment,
@@ -70,7 +70,16 @@ export function FileCard({
   onToggleViewed = () => {},
 }: FileCardProps) {
   const [expanded, setExpanded] = useState(true);
-  const commentCount = comments.length;
+
+  const commentCount = useMemo(
+    () =>
+      Array.from(commentsByLine.values()).reduce(
+        (sum, arr) => sum + arr.length,
+        0,
+      ),
+    [commentsByLine],
+  );
+
   const badge = changeTypeBadge[file.changeType];
 
   // Flatten all hunk lines for syntax highlighting
@@ -158,7 +167,7 @@ export function FileCard({
                   key={hi}
                   hunk={hunk}
                   filePath={file.newPath}
-                  comments={comments}
+                  commentsByLine={commentsByLine}
                   onAddComment={onAddComment}
                   onResolveComment={onResolveComment}
                   onUnresolveComment={onUnresolveComment}
diff --git a/apps/web/src/components/review/HunkRows.tsx b/apps/web/src/components/review/HunkRows.tsx
index 86cf6dd..9bec866 100644
--- a/apps/web/src/components/review/HunkRows.tsx
+++ b/apps/web/src/components/review/HunkRows.tsx
@@ -6,7 +6,7 @@ import type { LineTokenMap } from "./use-syntax-highlight";
 interface HunkRowsProps {
   hunk: { header: string; lines: DiffLine[] };
   filePath: string;
-  comments: ReviewComment[];
+  commentsByLine: Map<string, ReviewComment[]>;
   onAddComment: (
     filePath: string,
     lineNumber: number,
@@ -23,7 +23,7 @@ interface HunkRowsProps {
 export function HunkRows({
   hunk,
   filePath,
-  comments,
+  commentsByLine,
   onAddComment,
   onResolveComment,
   onUnresolveComment,
@@ -81,9 +81,9 @@ export function HunkRows({
 
       {hunk.lines.map((line, li) => {
         const lineKey = line.newLineNumber ?? line.oldLineNumber ?? li;
-        const lineComments = comments.filter(
-          (c) => c.lineNumber === lineKey && c.lineType === line.type,
-        );
+        // O(1) map lookup — replaces the previous O(n) filter
+        const lineComments =
+          commentsByLine.get(`${filePath}:${lineKey}:${line.type}`) ?? [];
         const isCommenting =
           commentingLine?.lineNumber === lineKey &&
           commentingLine?.lineType === line.type;
diff --git a/apps/web/src/components/review/ReviewTab.tsx b/apps/web/src/components/review/ReviewTab.tsx
index 099f380..8d0a2c0 100644
--- a/apps/web/src/components/review/ReviewTab.tsx
+++ b/apps/web/src/components/review/ReviewTab.tsx
@@ -7,6 +7,7 @@ import { DiffViewer } from "./DiffViewer";
 import { ReviewSidebar } from "./ReviewSidebar";
 import { ReviewHeader } from "./ReviewHeader";
 import { InitiativeReview } from "./InitiativeReview";
+import { buildCommentIndex } from "./comment-index";
 import type { ReviewStatus, DiffLine } from "./types";
 
 interface ReviewTabProps {
@@ -169,6 +170,11 @@ export function ReviewTab({ initiativeId }: ReviewTabProps) {
     }));
   }, [commentsQuery.data]);
 
+  const commentsByLine = useMemo(
+    () => buildCommentIndex(comments),
+    [comments],
+  );
+
   const createCommentMutation = trpc.createReviewComment.useMutation({
     onSuccess: () => {
       utils.listReviewComments.invalidate({ phaseId: activePhaseId! });
@@ -400,7 +406,7 @@ export function ReviewTab({ initiativeId }: ReviewTabProps) {
           ) : (
             <DiffViewer
               files={files}
-              comments={comments}
+              commentsByLine={commentsByLine}
               onAddComment={handleAddComment}
               onResolveComment={handleResolveComment}
               onUnresolveComment={handleUnresolveComment}
diff --git a/apps/web/src/components/review/comment-index.test.tsx b/apps/web/src/components/review/comment-index.test.tsx
new file mode 100644
index 0000000..c1aafb0
--- /dev/null
+++ b/apps/web/src/components/review/comment-index.test.tsx
@@ -0,0 +1,134 @@
+// @vitest-environment happy-dom
+import "@testing-library/jest-dom/vitest";
+import { render, screen } from "@testing-library/react";
+import { vi, describe, it, expect } from "vitest";
+import { buildCommentIndex } from "./comment-index";
+import type { ReviewComment } from "./types";
+
+// ── Stub CommentThread and CommentForm so LineWithComments renders without deps ──
+vi.mock("./CommentThread", () => ({
+  CommentThread: () => <div data-testid="comment-thread" />,
+}));
+vi.mock("./CommentForm", () => ({
+  CommentForm: vi.fn().mockReturnValue(<div data-testid="comment-form" />),
+}));
+vi.mock("./use-syntax-highlight", () => ({
+  useHighlightedFile: () => null,
+}));
+
+// ── Helpers ──────────────────────────────────────────────────────────────────
+
+function makeComment(overrides: Partial<ReviewComment> & { id: string }): ReviewComment {
+  return {
+    id: overrides.id,
+    filePath: overrides.filePath ?? "src/foo.ts",
+    lineNumber: overrides.lineNumber !== undefined ? overrides.lineNumber : 1,
+    lineType: overrides.lineType ?? "added",
+    body: overrides.body ?? "comment body",
+    author: overrides.author ?? "alice",
+    createdAt: overrides.createdAt ?? "2024-01-01T00:00:00Z",
+    resolved: overrides.resolved ?? false,
+    parentCommentId: overrides.parentCommentId ?? null,
+  };
+}
+
+// ── buildCommentIndex — pure function tests ───────────────────────────────────
+
+describe("buildCommentIndex", () => {
+  it("happy path — basic indexing", () => {
+    const c1 = makeComment({ id: "1", filePath: "src/foo.ts", lineNumber: 10, lineType: "added" });
+    const c2 = makeComment({ id: "2", filePath: "src/bar.ts", lineNumber: 5, lineType: "context" });
+    const map = buildCommentIndex([c1, c2]);
+    expect(map.get("src/foo.ts:10:added")).toEqual([c1]);
+    expect(map.get("src/bar.ts:5:context")).toEqual([c2]);
+    expect(map.size).toBe(2);
+  });
+
+  it("same-line accumulation — two comments land in same array", () => {
+    const c1 = makeComment({ id: "a", filePath: "src/x.ts", lineNumber: 20, lineType: "added" });
+    const c2 = makeComment({ id: "b", filePath: "src/x.ts", lineNumber: 20, lineType: "added" });
+    const map = buildCommentIndex([c1, c2]);
+    expect(map.get("src/x.ts:20:added")).toEqual([c1, c2]);
+    expect(map.size).toBe(1);
+  });
+
+  it("cross-type isolation — same lineNumber but different lineType produces separate entries", () => {
+    const added = makeComment({ id: "a", filePath: "src/x.ts", lineNumber: 10, lineType: "added" });
+    const removed = makeComment({ id: "r", filePath: "src/x.ts", lineNumber: 10, lineType: "removed" });
+    const map = buildCommentIndex([added, removed]);
+    expect(map.get("src/x.ts:10:added")).toEqual([added]);
+    expect(map.get("src/x.ts:10:removed")).toEqual([removed]);
+    expect(map.size).toBe(2);
+  });
+
+  it("null lineNumber — file-level comment stored under filePath:file", () => {
+    const fileComment = makeComment({ id: "f", filePath: "src/z.ts", lineNumber: null, lineType: "context" });
+    const map = buildCommentIndex([fileComment]);
+    expect(map.get("src/z.ts:file")).toEqual([fileComment]);
+  });
+
+  it("empty input — returns empty map", () => {
+    expect(buildCommentIndex([])).toEqual(new Map());
+  });
+});
+
+// ── LineWithComments — component tests ───────────────────────────────────────
+
+import { LineWithComments } from "./LineWithComments";
+import type { DiffLine } from "./types";
+
+const addedLine: DiffLine = {
+  type: "added",
+  content: "const x = 1;",
+  oldLineNumber: null,
+  newLineNumber: 5,
+};
+
+const noop = () => {};
+
+describe("LineWithComments", () => {
+  it("renders comment button with title when lineComments is non-empty", () => {
+    const lineComments = [
+      makeComment({ id: "c1", filePath: "src/foo.ts", lineNumber: 5, lineType: "added" }),
+    ];
+    render(
+      <table>
+        <tbody>
+          <LineWithComments
+            line={addedLine}
+            lineKey={5}
+            lineComments={lineComments}
+            isCommenting={false}
+            onStartComment={noop}
+            onCancelComment={noop}
+            onSubmitComment={noop}
+            onResolveComment={noop}
+            onUnresolveComment={noop}
+          />
+        </tbody>
+      </table>,
+    );
+    expect(screen.getByTitle(/1 comment/)).toBeInTheDocument();
+  });
+
+  it("does not render comment thread row when lineComments is empty", () => {
+    render(
+      <table>
+        <tbody>
+          <LineWithComments
+            line={addedLine}
+            lineKey={5}
+            lineComments={[]}
+            isCommenting={false}
+            onStartComment={noop}
+            onCancelComment={noop}
+            onSubmitComment={noop}
+            onResolveComment={noop}
+            onUnresolveComment={noop}
+          />
+        </tbody>
+      </table>,
+    );
+    expect(document.querySelector("[data-comment-id]")).toBeNull();
+  });
+});
diff --git a/apps/web/src/components/review/comment-index.ts b/apps/web/src/components/review/comment-index.ts
new file mode 100644
index 0000000..cc486ca
--- /dev/null
+++ b/apps/web/src/components/review/comment-index.ts
@@ -0,0 +1,25 @@
+import type { ReviewComment } from "./types";
+
+/**
+ * Build a Map keyed by `"${filePath}:${lineNumber}:${lineType}"` for line-level
+ * comments, or `"${filePath}:file"` for file-level comments (lineNumber === null).
+ *
+ * The compound key (filePath + lineNumber + lineType) is required because
+ * added and removed lines can share the same numeric position in a replacement
+ * hunk (e.g., old line 10 removed, new line 10 added).
+ */
+export function buildCommentIndex(
+  comments: ReviewComment[],
+): Map<string, ReviewComment[]> {
+  const map = new Map<string, ReviewComment[]>();
+  for (const comment of comments) {
+    const key =
+      comment.lineNumber != null
+        ? `${comment.filePath}:${comment.lineNumber}:${comment.lineType}`
+        : `${comment.filePath}:file`;
+    const existing = map.get(key);
+    if (existing) existing.push(comment);
+    else map.set(key, [comment]);
+  }
+  return map;
+}
diff --git a/apps/web/src/components/review/types.ts b/apps/web/src/components/review/types.ts
index 2b99452..2f9a1ad 100644
--- a/apps/web/src/components/review/types.ts
+++ b/apps/web/src/components/review/types.ts
@@ -28,7 +28,7 @@ export interface FileDiff {
 export interface ReviewComment {
   id: string;
   filePath: string;
-  lineNumber: number; // new-side line number (or old-side for deletions)
+  lineNumber: number | null; // null = file-level comment
   lineType: "added" | "removed" | "context";
   body: string;
   author: string;