The problem is that some devices somehow end up with a missing file that the Browser uses to talk to Flash Player. In my case if I clear the Browser data and reinstall the latest version of Flash Player from the market the config file does not get recreated.
I don't yet know what circumstances have led to the file not being created when flash is installed and why it seems to be generated on some roms but not others.
What I do know is that after adding this file, Flash starts working again in the Browser at 30fps.
If you only get 9-10 fps with the youtube speed test video then give this a try and report your findings in here.
If this works for you here's a link to some awesome videos from the guys who directed the batelco infinity video some of you may have seen.
Opera seems to still be laggy but this works for the Stock Browser and Dolphin HD
Flash this in CWM recovery or push the config.data file manually.
Here are patches that will fix either the Stock Browser or Dolphin HD