I've noticed that, too, but the benefits (less virus and spyware attacks) have been more than worth it. I wasn't aware that Firefox might run poorly on an older computer, mine is 3 gHz, and doesn't slow down for anything (except IE6 viruses!)
"...but the benefits..."
I agree. My criticism aside, I'd never go back to IE - too bloated and insecure (oops, I may have just described myself there).