I'm running TM 1.1b8 on a 400Mhz iMac DV/SE with 512MB of ram (running Tiger). Text rendering seems to be very slow... to the point where it really isn't usable. It's most noticeable when you're scrolling, or when you first open a document. It's slowest when syntax highlighting is on... otherwise it's not so bad.
FWIW, I'm seeing the same thing with b8 on an iMac G5 (Tiger, 1Gig RAM). Syntax-highlighted Ruby code just sort of paints itself down the document. It's fun to watch, but the novelty quickly wears off :)