I agree with squirrl. Worst case, set up your MacBook when you get home from work or over a weekend, and let FileVault run its course overnight. Once it's set up, it's hardly noticeable. The reason it is noticeable on the MacBook is that it is CPU intensive to encrypt a drive for the first time, particularly if it has lots of data, and the CPU needs to power down frequently in order to avoid overheating.
I'd rather have one day of inconvenience knowing that if I lose this, someone can't get at my data. Given how small this is, it might be easier than you realize. I also plan to get a personal articles policy on this (it came in handy once before when I left an 11.6" MacBook Air on an airplane).