Well, i believe that is just the average human reaction speed you are talking about. A trained fighter might be able to react at speeds twice as fast; not he would need to, as he can simply throw off the gunners aim beforehand, moving in zigzags and rolling between cover. There is no need to react to the shot if you use preventive measures that simply don't let them lock their aim on you.

Plus, i think we're talking about characters with various powers and/or gadgets that can help them further encumber their opponent's aim.