Apple: Please use the Firefox browser to sign up for WWDC 2010

Have you tried clearing the cache?

Browsers generally seem to have that issue, the longer you use them, the slower they get. When I started using Firefox, it was fast as hell, over the time it took at least a minute to start and caused beach balls all the time, taking up 900MB of ram sometimes with only 4 tabs open. Then I switched to Safari, was fast like the first day of using Firefox, but after a couple of months it was slow too.
I've been using Chrome now for about two months and there aren't any significant slow downs (yet).

Maybe it's just the cache growing, maybe it's the endless file of visited sites Firefox keeps or all the extensions (by which I mean 4). Seems like Safari is the fastest without extensions, but for me that makes it hardly useable, no adblock, no custom css…

But back to your original problem, maybe a little hardcore, but back up all Safari preferences (I don't know if there is more than /Library/Safari, maybe a Google search will help), delete the original ones and then see if it makes a difference. If it does, copy one file after the other and when you feel like it's getting slower you know which file it was, but deleting the cache should do.

Browsers could be so much more user friendly, considering they're used daily and probably more than most apps. It's great that there's a battle between companies, who can build the fastest browser, but I'd like them to compete for the browser that stays fast, I don't care if Safari can load java 5ms faster than Firefox if it takes twice the time to even start after using it for a month. But well, getting a little off topic here…

M$ might have to hire half of the population in India to work on IE tech support. lol

Thinking outside of the box (which is rare to see on the comments area like this) PERHAPS they need a lower level Browser for development on something they are working on to get it up to speed, just perhaps.