diff --git a/apps/readest-app/src/__tests__/utils/global-annotations.test.ts b/apps/readest-app/src/__tests__/utils/global-annotations.test.ts
new file mode 100644
index 00000000..81ce49bd
--- /dev/null
+++ b/apps/readest-app/src/__tests__/utils/global-annotations.test.ts
@@ -0,0 +1,126 @@
+import { describe, it, expect, vi } from 'vitest';
+import {
+  expandGlobalAnnotation,
+  removeGlobalAnnotationOverlays,
+} from '@/app/reader/utils/globalAnnotations';
+import { BookNote } from '@/types/book';
+import { FoliateView } from '@/types/view';
+
+/**
+ * Regression coverage for issue #4575: highlighting recurring character names
+ * (global annotations) made page turning very laggy. The `progress` effect
+ * re-fans-out every global annotation across every rendered section on EVERY
+ * page turn. Because the overlays are already drawn, re-walking the DOM and
+ * re-computing `view.getCFI()` for every occurrence is pure wasted work — the
+ * dominant per-page-turn cost on books with frequent highlighted words.
+ *
+ * `expandGlobalAnnotation` must therefore be idempotent: expanding the same
+ * note into the same (already-expanded) section a second time must do no work.
+ */
+
+const note = (overrides: Partial<BookNote>): BookNote => ({
+  id: 'note-1',
+  type: 'annotation',
+  cfi: 'home-cfi',
+  note: '',
+  text: 'foo',
+  style: 'highlight',
+  color: 'yellow',
+  global: true,
+  createdAt: 1,
+  updatedAt: 1,
+  ...overrides,
+});
+
+/** A jsdom section document where `text` recurs `count` times. */
+const makeDoc = (needle: string, count: number): Document => {
+  const doc = document.implementation.createHTMLDocument('section');
+  const body = Array.from({ length: count }, () => `${needle} `).join('gap ');
+  doc.body.innerHTML = `<p>${body}</p>`;
+  return doc;
+};
+
+type FakeView = FoliateView & { getCfiCalls: number };
+
+/**
+ * Minimal stand-in for the foliate view. `getCFI` returns a unique value per
+ * call (so it never collides with the note's home anchor) and counts how many
+ * times it ran — our proxy for the expensive per-occurrence work a page turn
+ * would repeat.
+ */
+const makeView = (docs: Document[]): FakeView => {
+  const overlayer = { add: vi.fn(), remove: vi.fn() };
+  const view = {
+    getCfiCalls: 0,
+    renderer: {
+      getContents: () => docs.map((doc, index) => ({ index, doc, overlayer })),
+    },
+    getCFI(_index: number, _range: Range) {
+      this.getCfiCalls += 1;
+      return `cfi-${this.getCfiCalls}`;
+    },
+    dispatchEvent: () => true,
+  };
+  return view as unknown as FakeView;
+};
+
+describe('expandGlobalAnnotation idempotency (issue #4575)', () => {
+  it('expands every occurrence on first call for a section', () => {
+    const doc = makeDoc('foo', 3);
+    const view = makeView([doc]);
+    const added = expandGlobalAnnotation(view, note({}), doc, 0);
+    expect(added).toHaveLength(3);
+    expect(view.getCfiCalls).toBe(3);
+  });
+
+  it('does NO work when re-expanding the same note into the same section', () => {
+    const doc = makeDoc('foo', 3);
+    const view = makeView([doc]);
+    const n = note({ id: 'same-note' });
+    expandGlobalAnnotation(view, n, doc, 0);
+    expect(view.getCfiCalls).toBe(3);
+
+    // Second pass simulates the next page turn — overlays already exist.
+    const added2 = expandGlobalAnnotation(view, n, doc, 0);
+    expect(added2).toEqual([]);
+    expect(view.getCfiCalls).toBe(3); // unchanged: no re-walk, no re-getCFI
+  });
+
+  it('re-expands when the note content changes (edit/recolor bumps updatedAt)', () => {
+    const doc = makeDoc('foo', 2);
+    const view = makeView([doc]);
+    const n = note({ id: 'edited-note', updatedAt: 10 });
+    expandGlobalAnnotation(view, n, doc, 0);
+    expect(view.getCfiCalls).toBe(2);
+    const added = expandGlobalAnnotation(view, { ...n, updatedAt: 11 }, doc, 0);
+    expect(added).toHaveLength(2);
+    expect(view.getCfiCalls).toBe(4);
+  });
+
+  it('re-expands after overlays are removed (toggle global off then on)', () => {
+    const doc = makeDoc('foo', 2);
+    const view = makeView([doc]);
+    const n = note({ id: 'retoggle-note' });
+    expandGlobalAnnotation(view, n, doc, 0);
+    expect(view.getCfiCalls).toBe(2);
+
+    // Toggle off: overlays torn down and the memo cleared.
+    removeGlobalAnnotationOverlays(view, n);
+
+    // Toggle back on with identical content — must re-fan-out, not short-circuit.
+    const added = expandGlobalAnnotation(view, n, doc, 0);
+    expect(added).toHaveLength(2);
+    expect(view.getCfiCalls).toBe(4);
+  });
+
+  it('re-expands into a freshly rendered section (different doc)', () => {
+    const docA = makeDoc('foo', 2);
+    const docB = makeDoc('foo', 2);
+    const view = makeView([docA, docB]);
+    const n = note({ id: 'multi-section' });
+    expandGlobalAnnotation(view, n, docA, 0);
+    const addedB = expandGlobalAnnotation(view, n, docB, 1);
+    expect(addedB).toHaveLength(2);
+    expect(view.getCfiCalls).toBe(4); // 2 per distinct section
+  });
+});
diff --git a/apps/readest-app/src/app/reader/utils/globalAnnotations.ts b/apps/readest-app/src/app/reader/utils/globalAnnotations.ts
index 7efd801e..3ecb655b 100644
--- a/apps/readest-app/src/app/reader/utils/globalAnnotations.ts
+++ b/apps/readest-app/src/app/reader/utils/globalAnnotations.ts
@@ -174,6 +174,28 @@ interface OverlayerLike {
   remove: (value: string) => void;
 }
 
+/**
+ * Per-section memo of which global notes have already been fanned out.
+ *
+ * Issue #4575: the `progress` effect re-expands every global annotation across
+ * every rendered section on EVERY page turn. The overlays already exist after
+ * the first pass, so re-walking the DOM and re-computing `view.getCFI()` for
+ * each occurrence is pure wasted work — and on books where highlighted words
+ * (e.g. recurring character names) appear hundreds of times per chapter it was
+ * the dominant per-page-turn cost (tens of ms of synchronous main-thread work
+ * each turn, multiplied on slower mobile hardware).
+ *
+ * Keying on the live section `Document` makes invalidation automatic: a section
+ * that is unloaded and later re-rendered gets a fresh `Document`, so its
+ * overlays are rebuilt; the stale entry is collected with the old document. The
+ * signature embeds `updatedAt` (bumped on every edit/recolor/global toggle) so
+ * an edited note re-expands once the caller has torn down its old overlays.
+ */
+const expandedByDoc = new WeakMap<Document, Map<string, string>>();
+
+const annotationSignature = (note: BookNote): string =>
+  `${note.updatedAt ?? 0}:${note.style ?? ''}:${note.color ?? ''}:${note.text ?? ''}`;
+
 /**
  * For a single `note` with `global=true`, fan out one overlay per
  * occurrence of `note.text` in the given section.
@@ -212,6 +234,12 @@ export function expandGlobalAnnotation(
   if (!live) return [];
   const { overlayer } = live;
 
+  // Skip the (expensive) re-fan-out when this exact note has already been
+  // expanded into this section's current document — see `expandedByDoc`.
+  const signature = annotationSignature(note);
+  let docMemo = expandedByDoc.get(doc);
+  if (docMemo?.get(note.id) === signature) return [];
+
   const added: string[] = [];
   const seen = new Set<string>();
   let occ = 0;
@@ -253,6 +281,17 @@ export function expandGlobalAnnotation(
       console.warn('Failed to add global annotation overlay', { note: note.id, err });
     }
   }
+
+  // Record that this note (at its current content signature) is now expanded
+  // into this section's document, so later page turns short-circuit above.
+  // Recorded even when there were zero matches — a note that doesn't appear in
+  // this section must not be re-walked every turn either.
+  if (!docMemo) {
+    docMemo = new Map<string, string>();
+    expandedByDoc.set(doc, docMemo);
+  }
+  docMemo.set(note.id, signature);
+
   return added;
 }
 
@@ -309,11 +348,15 @@ export function removeGlobalAnnotationOverlays(
   if (!view) return;
   const sections = (view.renderer?.getContents?.() ?? []) as Array<{
     index?: number;
+    doc?: Document;
     overlayer?: OverlayerLike;
   }>;
   for (const section of sections) {
-    const { index, overlayer } = section;
+    const { index, doc, overlayer } = section;
     if (typeof index !== 'number' || !overlayer) continue;
+    // Drop the expansion memo so a later re-add re-fans-out even if the note's
+    // content signature is unchanged (e.g. toggle global off then on again).
+    if (doc) expandedByDoc.get(doc)?.delete(note.id);
     for (let i = 0; i < maxOccurrencesPerSection; i += 1) {
       const value = makeSyntheticValue(note, index, i);
       try {