No problems here but I'm on a beefy machine: FF11, 64 bit Win7, quad core i5, 8GB ram, SSD drive - frankly I'd cry if a website was able to slow/crash that.
I use chrome on an 64 bit Win7, i7 2600K with 16GB ram and a 35/35 FIOS connection and plenty of flash sites slow things down to a crawl. Honestly, I cant help but wonder what exactly some developers expect people to use.
I don't think it's a question of what they expect. More like they have no idea how to program and Flash has let them get away with it. Seriously, there is no other sane reason why you should experience slow downs on a machine with that spec (assuming Flash can even harness most of that power).