I’ve always found Safari to be slower than other browsers, especially if it’s been running for a while. It also slows down other applications as well. Macenstein confirmed this, with some figures to back it up. Dave Hyatt gives some insight into what could be causing the slowdown. On the other hand, I find that Camino & Firefox also have the same effect.
A few days ago I tried Opera and found that it doesn’t slow down as much and it uses much less CPU time, so doesn’t slow down other applications. However I finally got annoyed with its quirkiness last night and switched back to Firefox. I found that using an unfamiliar browser and missing the extensions I use regularly slowed me down and made it more difficult to work.