Increase threshold for usage of compute for utf8 decoding on large strings to 50 KB (#64350)

On a local benchmark, removing compute is faster than compute in both debug and release mode on a Pixel 4. On a MotoG4, a much larger text sample (800 Kb) takes 50 ms, so we cannot simple remove the limit. Increase to 50 Kb which should take at most a ms or two on an older device, and only microseconds on newerones.

packages/flutter/lib/src/services/asset_bundle.dart

@@ -71,11 +71,13 @@ abstract class AssetBundle {
     // that the null-handling logic is dead code).
     if (data == null)
       throw FlutterError('Unable to load asset: $key'); // ignore: dead_code
-    if (data.lengthInBytes < 10 * 1024) {
-      // 10KB takes about 3ms to parse on a Pixel 2 XL.
-      // See: https://github.com/dart-lang/sdk/issues/31954
+    // 50 KB of data should take 2-3 ms to parse on a Moto G4, and about 400 μs
+    // on a Pixel 4.
+    if (data.lengthInBytes < 50 * 1024) {
       return utf8.decode(data.buffer.asUint8List());
     }
+    // For strings larger than 50 KB, run the computation in an isolate to
+    // avoid causing main thread jank.
     return compute(_utf8decode, data, debugLabel: 'UTF8 decode for "$key"');
   }