Thanks for the replies.
Let me clarify a bit my situation. I'm not trying to play them at the same time, I'm just saying that streaming the same files to my ps3 has no problem at all, to my xbox 360 (not at the same time) it seems like the framerate is like 17 fps or something (not actually measured but that's how it feels like, not super choppy but not entirely fluent either, audio is fine btw). Also I'm talking about standard definition files here, 624 x 352. My CPU is a E6750, not the greatest but transcoding 720p even to my ps3 runs perfectly with cpu power to spare, so 624 x 352 really shouldn't be a problem for my processor.
So it's probably a setting somewhere, I'm taking PMS doesn't transcode to the same format when streaming to ps3 or xbox360 ?