diff --git a/.jules/bolt.md b/.jules/bolt.md
index d54cf10fc..3678af523 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.
+
+## 2026-07-15 - Replace Array.every for large byte arrays
+**Learning:** Using `Array.prototype.every()` on very large arrays (like serialized PDF byte buffers) introduces massive function call overhead for each element, leading to severe main-thread blocking.
+**Action:** Use a standard `for` loop with an early return to check large byte arrays, avoiding O(N) callback allocations.
diff --git a/apps/desktop/src/features/score/scoreStorage.test.ts b/apps/desktop/src/features/score/scoreStorage.test.ts
index 0feec199e..f58e94dc0 100644
--- a/apps/desktop/src/features/score/scoreStorage.test.ts
+++ b/apps/desktop/src/features/score/scoreStorage.test.ts
@@ -32,4 +32,21 @@ describe("scoreStorage bridge resolution", () => {
       BRIDGE_UNAVAILABLE_MESSAGE
     );
   });
+
+  it("correctly reads a score pdf returning a number array", async () => {
+    const tauriWindow = window as TauriWindow;
+    tauriWindow.__TAURI_INVOKE__ = vi.fn().mockResolvedValue([1, 2, 3]);
+
+    const result = await readScorePdf("project-1", "score-1");
+    expect(result).toEqual(new Uint8Array([1, 2, 3]));
+  });
+
+  it("rejects reading a score pdf when returning an array with non-numbers", async () => {
+    const tauriWindow = window as TauriWindow;
+    tauriWindow.__TAURI_INVOKE__ = vi.fn().mockResolvedValue([1, 2, "not a number"]);
+
+    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..7f938fc02 100644
--- a/apps/desktop/src/features/score/scoreStorage.ts
+++ b/apps/desktop/src/features/score/scoreStorage.ts
@@ -91,8 +91,21 @@ 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 Array.every() and Uint8Array.from() to prevent O(N) callback overhead
+    // and intermediate array allocations when parsing potentially large PDF byte arrays.
+    const uint8Arr = new Uint8Array(response.length);
+    let allNumbers = true;
+    for (let i = 0; i < response.length; i++) {
+      if (typeof response[i] !== "number") {
+        allNumbers = false;
+        break;
+      }
+      uint8Arr[i] = response[i] as number;
+    }
+    if (allNumbers) {
+      return uint8Arr;
+    }
   }
 
   throw new Error(INVALID_RESPONSE_MESSAGE);