Don't hide your mistakes or lack of good techniques by using a lot of speed. In the Lap sau or grabbing hand exercise, people often perform the grab and hit with high speed during the sticking hands training. This looks very impressive to the outsider, but not to the Wing Chun expert; because the technique is often done incorrectly.
In the Lap sau exercise, the grabbing hand should have the elbow and forearm along the vertical center line for protection. If the elbow is moved out during the grab, then you are open to a center hit. An alert Wing Chun practitioner will apply a center hit, which is always faster than the grabbing hit technique. Therefore, train in accurate positions first and try for speed later. Remember, a good opponent will counter sloppy actions easily.