Boy, I would be shocked if full-drive encryption didn't bring your processor to it's knees. I've tried it on a dual-core machine before and it has been very slow.

Have you looked at other options? Depending on the amount of data that needed to be encrypted, you might look at something like an Iron Key (fully encrypted USB device). I have one and it's great because it automatically encrypts when it's removed or the machine is shut down, plus it decrypts easily when you want it to....