Coleman is clearly an outstanding athelete, but he lacks the all important attribute of superior natural speed...
I find it very hard to judge pure speed on the field due to player smarts, instincts, etc. In any event, I would say look back at the last two year's bad plays by Coleman and you will find that over-aggression, pure being out of place and slipping (ahem Titans game last year) account for the vast majority of his "burned on" plays rather than straight up speed. Go back and look at the tape and find the instances of pure speed rather than bad judgment being the cause--they are few and far between. Coleman gambles and/or get's distracted at time--the former leads to INT's as well but both contribute to giving up plays. His speed is sufficient especially in the age of "who cares if he runs a 4.6 or 4.7 40 he is a great WR." All that said, Coleman has the potential to be a great, ball-hawking FS which in our division could be huge.