[Bug]: Buffer unable to keep up with playback with Prores files

### What happened?

When playing back a prores file, the look ahead buffer is unable to keep up with realtime playback (even if it exceeds the filesize of the file being played back). It is really bad with multiple Reader Threads (under Preferences -> Caching) and slightly better with 1 reader thread. However it is unusable to playback even a short file.

### List all the operating systems versions where this is happening

macos 26

### On what computer hardware is this happening?

mac air

### Relevant console log output

```shell

```

### Environment variables

_No response_

### Extra information

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The report names no source file, test, or entry point. Reproduce on macOS 26 with a ProRes file on a MacBook Air, comparing one and multiple reader threads in Preferences → Caching; done means playback keeps up with real time.

Written by the indexing model from the issue text.