Footspeed isn't everything. Shavon isn't all that quick, either. But he's smart, and he anticipates where people want to go. The best defenders are players that know their opponents tendencies and act on those decisively. That's a mental function. It's also about discipline: trusting your scouting reports, and staying on those instead of getting happy feet. I've seen plenty of blazing fast players suck on defense because they're not mentally geared to stopping their opponents.
Look how quick Biggs is, and how awful he could be defensively.