The initial encryption will suck. Walk away for a few hours.

After that, I don't think it'll be that bad (the I/O subsystem in those things is already horridly slow--the encryption overhead is not really going to add enough to be noticeable).

Plus, the truecrypt folks just (well, a few months ago) re-implemented their AES routines in assembly (actually, they borrowed the implementation). But it made a drastic difference in speed on my personal laptops.