Here's an interesting news video i found about ADHD and finger tapping. It doesn't talk about speed, but talks about the fact that with ADHD kids, the fingers on the opposite hand move as well (esp in boys). I wasn't looking for this when I asked my two to try it.

http://abcnews.go.com/Health/Wellness/mirror-movements-reflect-adhd-kids/story?id=12919887