His 10 yard split was top ten for DT , he was 3rd in vert and 6th in broad jump which is more indicative of burst and at his size is pretty amazing. Getting off at the snap of the ball is all mental processing, if you are asked to first read the play and then react you are much more likely to be slow to start than if you are looking to just tear ass to the QB at the snap.
Also, an observation at yesterdays rookie camp was he was quick off the ball early and slowed later, so the excessive reps at UF and his need for better conditioning likely played a huge role, and we know the Bears are going to be big on being in top condition.