I'm getting the same issue. Flashes. Very slow to load and sometimes it becomes unresponsive for 5-10 seconds. I don't get the white blocks though. Cannot fully load that page without something going wrong.
I'm not getting white blocks, but when playing youtube videos, I do get a quick white flash on the video screen and then it keeps right on playing back to normal. These were videos that were favorites I had on youtube and I never had any white flashes on them on my iPad 2.
No light leaks or light bleed at all for me. The iPad 3 though does seem like it could use more memory. When I go into Settings and then start clicking on General, then iCloud, then Mail, then Twitter, etc, it's instantaneous on some of them, and others, there's a noticeable 1-2 secong lag sometimes as it switches to the different fields. Never had this on the iPad 2.
Still, overall I'm very very happy with the upgrade. The dictation feature is way better, and more accurate, than I thought it would be too. I can see myself using this feature a lot. I think they could have done more with the iPad 3 (although the screen is outstanding), I just expected a little more of at least 1 "wow" factor, and really, there was none. It's a much better upgrade if you're going from an iPad 1 or never owned an iPad, than going from an iPad 2. I think I easily could have stayed with the iPad 2 and been happy, but it didnt cost that much after selling my iPad 2 to upgrade to the new/3.