Obviously you cant pinpoint the exact speed. But say you watch Lance Moore, you know he is not a guy with 4.3 speed on the field. You watch Mike Wallace and you know he aint running a 4.6 out there.
Just because you run a fast 40 doesn't mean it translates to the field. Its all about how fast you run routes, make short adjustment depending on coverage, and how you get out of your breaks.
Stills is not slow obviously, he just doesnt showcase what you would think you would see from a guy who runs a 4.3 on a track.
Maybe it was how his offense operated, who knows. But im sure he can still get behind a defense once he learns how to beat press and make smooth transitions.