PCWorld did some tests to find out just how much of a difference running Flash makes.
In a word: significant.
Flash also consumed a significant amount of memory with both browsers.
Firefoxs benchmarks werent so dramatic, but the experience was the same:
Firefox?
Well, Firefox did swimmingly as well, when you removed Flash from the equation.
It consumed 1.65 GB, and needed only 24.5 percent of the CPU.
But after I downloaded Flash, Firefox seemed to throttle itself.
That was totally unacceptable.
In the meantime, you might test for yourself by going to chrome://plugins and disabling Flash.
Tested: How Flash destroys your browsers performance| PCWorld
