I think that your problem is not how fast you retrack your hand, but your positioning. If you are in bad position and throw the jab and i can sence the jab coming i am going to hit you every single time with my cross! And most likely i will land the punch when your jab is still going forward There are 2 things that you can work on. Improuve your stance and perception of positioning and don't telegraf the punch. Ask your self this questions. Are you jabing across? Is your head on the line of fire of his right hand? Are you squared? If the answer is yes to all of them than bringing the hand faster back wont improuve your situaion.