diff --git a/.jules/bolt.md b/.jules/bolt.md
index d54cf10fc..9edf08b3a 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -61,3 +61,7 @@
 ## 2026-07-13 - Array.from mapping optimization
 **Learning:** Using `Array.from({ length: N }).map(...)` creates an intermediate array of `undefined` values which requires memory allocation and garbage collection, adding O(N) unnecessary overhead in frequently re-rendered UI components.
 **Action:** Use `Array.from({ length: N }, (_, index) => ...)` to map elements directly during array creation, avoiding intermediate allocations.
+
+## 2026-09-26 - O(N) Callback overhead on massive arrays
+**Learning:** Using `.every()` on massive arrays (e.g., megabyte-sized PDF byte buffers) incurs severe callback overhead and blocks the main thread.
+**Action:** Replace `.every()` and similar array methods with a traditional `for` loop for massive arrays to achieve a measurable optimization and prevent main-thread blocking.
diff --git a/apps/desktop/src/features/score/scoreStorage.test.ts b/apps/desktop/src/features/score/scoreStorage.test.ts
index 0feec199e..dae73af10 100644
--- a/apps/desktop/src/features/score/scoreStorage.test.ts
+++ b/apps/desktop/src/features/score/scoreStorage.test.ts
@@ -33,3 +33,27 @@ describe("scoreStorage bridge resolution", () => {
     );
   });
 });
+
+  it("handles valid arrays of numbers", async () => {
+    vi.stubGlobal("window", {
+      __TAURI_INTERNALS__: {
+        invoke: async () => [1, 2, 3],
+      },
+    });
+
+    const result = await readScorePdf("project-1", "score-1");
+    expect(result).toBeInstanceOf(Uint8Array);
+    expect(result).toEqual(Uint8Array.from([1, 2, 3]));
+  });
+
+  it("throws error when array contains non-numbers", async () => {
+    vi.stubGlobal("window", {
+      __TAURI_INTERNALS__: {
+        invoke: async () => [1, "2", 3],
+      },
+    });
+
+    await expect(readScorePdf("project-1", "score-1")).rejects.toThrow(
+      "Invalid score bridge response"
+    );
+  });
diff --git a/apps/desktop/src/features/score/scoreStorage.ts b/apps/desktop/src/features/score/scoreStorage.ts
index 492f12591..d80dc0d62 100644
--- a/apps/desktop/src/features/score/scoreStorage.ts
+++ b/apps/desktop/src/features/score/scoreStorage.ts
@@ -91,8 +91,19 @@ export async function readScorePdf(projectId: string, scoreId: string): Promise<
   if (response instanceof ArrayBuffer) {
     return new Uint8Array(response);
   }
-  if (Array.isArray(response) && response.every((byte) => typeof byte === "number")) {
-    return Uint8Array.from(response as number[]);
+  if (Array.isArray(response)) {
+    // Performance: Avoid massive O(N) callback overhead of .every() on large byte arrays (e.g. megabyte-sized PDFs).
+    // A traditional for loop significantly reduces main-thread blocking time during validation.
+    let isValid = true;
+    for (let i = 0; i < response.length; i++) {
+      if (typeof response[i] !== "number") {
+        isValid = false;
+        break;
+      }
+    }
+    if (isValid) {
+      return Uint8Array.from(response as number[]);
+    }
   }
 
   throw new Error(INVALID_RESPONSE_MESSAGE);