I used to think Firefox was pretty fast, but that starts to become a long time ago.... Until I installed the Flashblock plugin.
With flashblock installed, the flash content on web pages is blocked and displayed as an empty square with an "f" button in the middle. If you really want to see the useless advertisement or graphic in the flash, just click on the "f".
After installing flashblock, it has suddenly become obvious how pervasive (and destructive to surfing speed) flash content has become. It seems to be everywhere, and as far as I can tell, the content is universally uninteresting. Certainly not worth the performance hit.
Flash bites you in two places. The obvious one is bandwidth. Flash objects tend to chew up a lot. With flash blocked, many web pages with several flash objects download in a fraction of the time. (This is particularly critical here in the bandwidth-starved part of the world, which tends to be most places outside of N.America and Europe....)
The second place the flash bites you is in the CPU, particularly if you have a somewhat older / slower machine. Flash objects tend to be resource hogs and will make your browser slower, and even appear to slow down your whole machine if you leave some flash running in your browser while you go and do some other things.