I'm fairly sure you can't watch Youtube vids on PS3, because of the Flash plugins that are needed. So it's probably trying to load a 5MB web page (or whatever), which it doesn't know what to do with. I've never tried. In fact, I rarely use the browser at all.. it's clunky as hell with a controller.
most youtube videos play very well, just some pages overlaod the system memory i guess. Probalby about %5 of them. I think the browser is actaully really easy to use, typing is the worst part