PS3 doesn't support mkv and for mkv files tversity for example actually transcodes the mkv to mpeg2 on the fly. Transcoding is the reason why only 1.5 fast forward works. And at that point quality is lost and loads of cpu power is used. I prefer to use ps3video9 to remux mkv to .mp4 because of better quality and no need for transcoding.
Remuxing in itself doesn't take much power to do quickly; it'd be nice if there were some sort of middle man that could take a streamed mkv and serve it on the fly as mp4 to the PS3 (if the format itself is capable, anyway). Same thing for profile modification on avc streams and subtitle rendering.