My experience with all 3 of my Sigma lenses is that the AF is slow if it has trouble finding your intended target. They like to back all the way out then start focusing from zero. The one time I rented a really nice Nikkor for a sporting event, I could notice a significant difference in being able to focus on moving subjects fast. But on the other end, if they don't need to back out to find the subject, then the AF is super fast.
I have the 100-300 F/4, 150 2.8 Macro, and 17-70 2.8-4.