Discussion on the topic here, if you'd like to see the current progress on figuring it out:
I've successfully stopped all WalkSpeed changes, allowing a 16 walkspeed max, thus stopping "some" speed exploiting.
My problem is, however, that there is a new way to "speed exploit", by adjusting the animation speed. I've been researching animations, animationtracks, animation speed, etc., though I've not found a way to locate the AnimationTrack if you weren't the one to declare it.
This poses a problem, as I cannot index the AnimationTrack to find when it changes, and, upon changing, reset the animation speed to be the default speed.
I've been looking at everything I can find on the subject, though I've not been successful.
If anyone can find where to locate it, so I can effectively stop speed exploiting, I'd be greatly appreciative.
I think you're seeing exploiters who are increasing their Physics FPS. Here's a tutorial on detecting them:
http://wiki.roblox.com/index.php?title=Stopping_speed_hackers