My dad has the same issue, he's too slow, mostly because he has line management problems and has all these C loops to pick up before getting to the fish. Thats another problem, we all say to fish/set the hook upstream for the better hooking angle. But the pull comes from the direction of the line. If you're fishing straight across, i.e. across current, and you have a big C-loop in the current, the pull on the fly will come from downstream. It'll pull the fly right out of its mouth. You want small S-loops for drag control, but as little "extra" line out as possible.
But lots of people are too fast, including me at times. It's not like spin fishing where you don't know you have a bite till you feel it. You see the hit right away, and often before it even happens as the fish approaches. Many times I've anticipated the hit and yanked the fly away from the fish before it even makes contact, especially on those slow, leasurely "sip" type risers.
You want the fish to take, close its mouth, and turn down before setting the hook. But no longer than that, or it will spit it as it'll spit any artificial. It takes a feel, keep at it, you'll get it and it will become natural.