Obviously having a separate queue structure with all the video data being memcpy-ed an extra time (or is it two extra times?) is going to add to the processing load.
So there will be files that can play without the extra buffering that won't play with the extra buffering.

I'm not sure if this is the only effect @arnova is seeing, or if the buffering provokes some other behaviour (e.g. locking or thread starvation effect producing a different bottleneck).

So, is the CPU maxing out at 100% when buffering is enabled? What is it on same file with buffering disabled?