imo it's a combination of speed and agility that help you close in. DT's have a short area to cove to make plays so you want to get up to top speed as fast as possible, accelereation is determined by agility. If you have speed at say, 15 and agility at say, 40 you will be up to full speed right after you beat the lineman but not move fast enough to get there. If you have speed at 40 and agility at 15 you wont reach top speed before the play ends. Find that magic number that allows you to get through the line and burst to top speed right before making the hit. I think agility should be about 20-30% higher than speed, keep in mind that agility helps you break through blocks and helps to activate some special abilities.