That's odd... mine was lighning fast. More to do with the body you used it with perhaps?
Using a 1D and its definitely a tab slower on AI Servo. It's still fast but I did notice a difference taking some jet ski pictures in the summer. I love it being black too. Mind you I was using a CPol when I noticed this slight hesitation so it might have caused the slightly slower focus, so it did hunt a bit more with the lower light.. Its a beautiful lens and quality in my opinion doesn't get any better.