First make sure you're in an area with good signal strength, even though the hspa network is great if you are in a spot with poor cellular coverage it will still take longer to buffer a video. If that doesn't work then try the suggestions by the previous poster Posted via BlackBerryForums.com Mobile
Very good recommendation, I've actually found if you do not clear the browser cache, eventually you get browser messages stating out of memory. (or something along those lines) (I've heard this from many of my BES users, running 9650/8350i's/etc)
Of course I can't remember the last time I cleared my browser, but I find it typically the easiest step to perform when having browser issues.
8830 -> 8330 -> 9550 -> 9650
Just think about how far BlackBerries have come from then till now... And what else is coming.