Why does Chrome video performance substantially degrade after waking from suspend in 10.10?
Solution 1:
Upgrading to the new Chrome 8 beta fixed my problem with HTML5 video completely, since it added GPU acceleration. Flash is still as slugish as ever, but it runs well enough in Firefox, which isn't a problem for me since I've been using Firefox as my "crap, I need Flash for this" browser for a while anyway (I've had Flash disabled in Chrome 99% of the time for a while).
This isn't really an answer so much as an admission of defeat, but pending an actual solution, I think it's probably the most sensible advice for most people.