the mag spring is pushing up on either the follower or the rounds in the mag up against the bottomof the slide. When you push the mag release, that pressure is relieved by forcing the mag downwards. This is why if the slide is open, the mag may or may not pop out of the gun with as much force as normal. If you have slow to release mags, they may be dragging on something. For example, some of the glock mags will get deformed on the front sometimes right at the top of the mag. The polymer will get pooched out by bullets feeding from the magazine pushing the poly forwards. This will slow down or retard the mag drop. To solve that one, just take a sharp knife & trim off the offending poly. Sometimes that will happen around the mag catch notch.
Give competition shooting a try... don't let yourself be scared off by the thought of how poorly you may do. It is a learning experience, and only those willing to swallow their egos and take the plunge will ever reap the rewards.