A browser doesn't seem like it should have that adverse of an effect on one's system performance. Aside from the power users who surf for hours at a time and leave their browser running for days on end, one's browser shouldn't be dragging the rest of the class down.
Macenstein, on the other hand, has used a few pseudo-scientific tests to determine otherwise. While the debate still rages as to which browser is the 'best,' or the fastest, or the least detrimental memory hog, Dr. Macenstein has apparently outed Safari as a fairly selfish system resource gobbler, able to slow at least some operations by 76 percent. The tests performed by the monster of all things Mac included a fairly tricked out G5 PowerMac, as well as Quad Core Mac Pro just for good measure. Just to round the tests out, Macenstein eventually added Camino and OmniWeb to the original test sequence of Safari and Firefox. These browsers were all tested separately against opening/saving a fairly hefty PSD in Photoshop, as well as rendering a project out of After Effects (remember: those two apps still have to run emulated in Rosetta on the Mac Pro).
The cliff notes results? In nearly every test, Safari (running in the background) was found to deal a significant blow to performance and efficiency, causing the three aforementioned operations to take noticeably longer. To make things even more bizarre, Safari was actually found to not affect performance when exporting a video for the iPod with QuickTime.
At the end of the day, no one is really sure why Apple's browser is making so many waves in the performance pool, but a healthy comment thread on the post is already hard at work. For anyone serious about Safari, here's hoping Apple is already aware of the issue and has brought their browser in line for Leopard.