You can only pick as fast as you can recoil. If your hand is slower at getting back to the position you need to be in to perform the downstroke, you will only be able to downstroke so fast. I would suggest practice ONLY upstrokes for a while when practicing. In time you willl notice your downstrokes to become quicker.
To decrease your range of motion, watch your picking hand and see how far your pick goes past the string after you hit it. Practice minimizing that distance and your downstrokes will get faster.
Hope this helps.
