It probably is hurting their times depending on what they are doing. The only time a vertical grip makes perfect sense is when shooting an M4 carbine in combat, especially full auto. Combat is different. You often have to shoot from cover. Pretty hard to shoot from cover if you have to have your off hand way out on the end of the tube. But if you are shooting steel, paper, whatever in an action match or in a static position match the only thing that counts is transition speed. In that case a VG will slow you down, even if you don't realize it. Of the current and former USAMU guys I know who shoot 3-gun and such none use a VG either (JJ Racaza, KC Eusebio).
I have a VG but I seldom use it. I have timed myself with various configurations and it is slower. Mainly in keeping the sight picture while moving from target to target. BJ Norris actually rolls his off hand almost onto the top of the tube at the end. Me I have a bad left wrist from a motorcycle roadracing crash back in 1975 and can't turn it over that far so I don't. Perception is often wrong. Without some kind of empirical test you can't know what is the best way. That is why those of us who compete, at whatever level, always own or have access to a timer. Because you might think one way is good and it might not be.
Never believe what anybody else does is correct unless you try it yourself. Somebody may be doing it one way for different reasons than you think. So you ask and then try it.
And just because it is better than what you did before does not make it the best way to do it, it is just better than what you did before. Things evolve all the time.