Operationally, the classic valve can perform faster than any player can manually pull the trigger, so performance is not an issue with it.
Short stroking was a common issue with classic valves caused by not fully releasing the trigger before pulling it for the next shot. Classic valves work great if you train yourself to have good trigger habits. That good habit often goes out the window in the heat of battle. The X-valve gives a bit more kick back on the trigger which eliminates short stroking, and the recharge rate is quicker. This means that it is more forgiving which allows you to fire faster without ideal trigger habits, even without tuning it to have a reactive trigger.
Except for the Automag in front, its usually the man behind the equipment that counts.