AnandTech does a nice job of breaking down the performance hits you get with FileVault. They note a 20-30% hit in performance, but I am hear to tell you that I haven’t noticed any such slow downs and some of their tests seem a bit unrealistic to me.
Their tests are things that an average to above-average user are likely never to do. ((Unless average users open 42 reply email windows at once. I don’t think I reply to that many emails in a day.))