There are several methods on how to drop the slide following a reload:
1) One way is to use the off hand to reach over the top, palm down, pulling the slide to the rear and letting go. This works fine, altho it takes some time to reestablish the two handed firing grip. There are advantages for this method, but speed isn't one of them.
2) Another way is similar, but instead of palm down, the rear of the slide is pinched between thumb and forefinger. It's not as sure as the first method and is still slow.
3) Another method is to use the gun hand thumb to release the slide stop. For most people, this requires a grip shift to reach the slide stop. This is considerably slower than Method #1.
4) The best method I have found is upon seating the fresh magazine, use the left thumb to activate the slide stop. As the slide is going forward, y9u've reestablished a two handed hold. When done right, there is a sensation of, "I wish the slide would hurry and lock up so I can shoot!"
Some may say that this last method requires fine motor skills, but didn't you just execute some "fine motor skills" hitting the mag release and getting the new mag out of the pouch? Now, a mere second or two later, you're suddenly incapable of such fine motor skills?
I do and teach both methods #1 and #4, with emphasis on #4