Quicktime not the best quicktime player?

So, I have been struggling lately to play these h.264 quicktimes on my oldish pc and old iBook. Usually it plays slow and jumpy and I just assumed that my processor couldn't handle the new high quality codec - but then (for a laugh) I tried VLC player and not only can it play quicktimes but it plays them PERFECTLY! No sluggishness or low frame rates!

I don't know how to explain this.