it seems very slow in auto focus on my XTI as it takes 2-3 seconds from 1:1 to infinity!!!
Just timed mine going from infinity-->1:1-->infinity. Around 5sec (so ~2.5sec from one end to the other).
Think that matches up with my experience when trying someone else's 100L too.
The macro lens has a lot further to travel than the 24-105, so you can't really compare them to each other like that. For non-macro shooting, turn the lock on and it speeds up considerably.