Quote From Tudor_Queen:
For anyone else interested, I've now looked into this quite a bit, and there doesn't seem to be any con to encryption. : )
With modern PCs you are probably correct here. The problem is slower access to your hard disk but if you have enough RAM (which you probably will have) then once the encrypted file is copied to RAM (presumably a one-off activity for the most part) and decrypted you should experience no further slow down. I would probably be a little concerned about performance using things like Word which has been notoriously bad for memory usage for decades. In the old days this used to require a lot of paging to and from the hard disk because so little RAM was left free.