HTML5 is hardly what it's cracked up to be, now. Why Apple backs it is it gives Apple an out to get people used to paying for media content, because if they get free content now, they'll want it forever. Flash provides free content now (paid for via advertising). Apple's business mode requires selling the content via iTunes, App Store, and iBookstore.
There was a study last week using both HTML5 and Flash. When both had access to h.264 hardware decoding, they both had the same results. Flash would be a hell of a lot better on your Mac if you booted it into Windows via Boot Camp. I use about 20% of the power in Windows to run the same video, HD, and Flash apps as the same hardware uses in OS X.
Basically, live with the lags unless you're willing to use Windows. OS X is not the OS for entertainment whether it be Flash content, Flash Apps, HD videos, or even streaming videos.