Comment 3 for bug 141390

It seems it isn't related to the file-size; that is just a coincidence.

I've found that MP3 tracks of any length that are 32kbps or 64kbps (mono or stereo) cause this whilst 128kbps tracks are okay. Most of the audio-books are 32 or 64 kbps although I found one that was 128kbps, and it played without the CPU load issue. Almost all my music files are 128kbps or higher.

The only thing I can think of to explain it is some kind of up-sampling although 32/64kbps to 128kbps isn't exactly intensive.