Think of it this way, when you have a normal system, your write is flushed out to the disk as fast as possible. When you add in full disk encryption, you're going to have to encrypt on write. Your write won't be acknowledged until it's physically on the disk, so the faster your disk is, the faster your write will be acknowledged. Same thing with reads, the faster you can read, the sooner you can decrypt and return the data to the application.

So SSD is definitely the way to go if you want to encrypt the whole disk. If that's too expensive, get a 7200rpm laptop drive.