diff --git a/test/thumbnails/thumbnails.test.ts b/test/thumbnails/thumbnails.test.ts
index 580a312..4acd89b 100644
--- a/test/thumbnails/thumbnails.test.ts
+++ b/test/thumbnails/thumbnails.test.ts
@@ -787,12 +787,13 @@ describe("fixMissingThumbnails", () => {
     });
 
     it("re-encodes smaller until the thumbnail fits the recorded size", async () => {
-        // A noisy 400x300 JPEG, which the default encoding (quality 50, not
+        // A noisy 64x48 JPEG, which the default encoding (quality 50, not
         // resized because it is under 720 px) cannot compress below the size
-        // recorded here: one byte less than that encoding's ciphertext.
+        // recorded here: one byte less than that encoding's ciphertext. It is
+        // small so that each encode is quick even on a busy host.
         const fixMock = await buildThumbMock();
-        const w = 400;
-        const h = 300;
+        const w = 64;
+        const h = 48;
         const noisy = new Uint8Array(
             jpegJs.encode(
                 {