Re: Firefox + Flash terribly slow
I do think that this may be a flash issue mostly. The reason being, I use hulu (flash based) on my HTPC. Flash full screen pegs the CPU, however, keeping the flash window normal, turning on Compiz, and using the compiz zoom on the flash window works well.
Using MythVodka (MythTV Hulu Plugin that unfortunately no longer works) resulted in a great fullscreen experience (keep in mind that mythvodka had to read the flash video, convert it on the fly to some other video standard and then play and full screen that).
So playing flash and using anything other than flash's built in full screen works. I know that flash versions up to 9.24? (version might be slightly off) worked better. From 9.112 (when they changed their fullscreen code) onwards things went to heck