A well designed thumb break won't slow you down all that much. The trick is to train using it. I have found that many will go to the range and unsnap the break when they reach the line. They won't resnap until they leave the line. If anything happens on the street the only thing they have practiced is pulling the gun out of the holster and firing it; they will flub the draw. Practice with the break snapped before the draw (draw from a snapped holster). I use only one holster that does not have a thumb break and that is a Fobus paddle for a SIGPRO SP2340. All my other holsters have a thumb break. Which ever you choose practice with it concealed and exposed. Go slow at first, make a fluid motion (smooth as a baby's oiled bottom), the speed will come as a result of the smooth motion. One other thing practice the draw with the gun unloaded. There are many stories of the guy who practiced his "fast draw" only to shoot himself in the foot or somewhere else; or shoot a TV set, mirror, or put a hole in the wall. Stay safe.