I beat the crap out of my systems and I can't see any slow down in my laptop when I use LUKS, AES 256, and LVM as specified in the Slackware distribution docs.
I stayed away from disk encryption for a long time because I am always trying to eek out as much performance as possible. I use Fluxbox for my Window Manager, KDE is a pig. This is a 2.2 GHz dual Core Intel Centrino with 4 Gigabytes of RAM. It's a decent system but there is seriously no performance difference as far as I can tell. Things have been rock solid and reliable. Suspend works as well. I should have encrypted it a long time ago. The speed difference was the boogy man. I can't see that it exists.
The benefits of data safety are obvious. I wish I had tried this sooner. I would have slept better when traveling with this thing. AES appears to be "Good Enough" for Top Secret government stuff, so I guess I'll live with it ;-) Top Secret requires AES 192 or something.