IT'S FINE. ALL OF MY GLOCKS DO THIS!!!!!!
Sometimes, but not all of the time. And M&P, and HK, and Walther, and Ruger, and most of the other striker pistols. I currently have one old G23.3, one new G23.4, a brand new G41.4, and a brand new (less than 200 rounds) G34.4 that all do this.
Glocks don't cycle that slow. It's unnatural to gently ride the slide home that slowly. Momentum is what closes that last bit of travel when the trigger is held back. Chances are Glock put in a new striker spring. Fresh new striker springs can be a bit stiff.
I test the recoil spring by bringing the slide back a little, and letting go gently with my finger off of the trigger. It's a stupid test. If you feel the need for this test, don't bother, just put a new recoil spring in. They're cheap, and critical.
With your finger on the trigger, the recoil spring has to overcome the striker spring as well. When you lift off the trigger, the striker can relax a little, and the slide rides home. PERFECTLY NORMAL. It most often happens with a mag in and a snap cap in the bore (press check). It happens less often on Glocks with 5000+ rounds through them.
Mine will all do it all the time, with a snap cap in, and another snap cap in the mag.
The OP likely already has a new spring. Glock swaps them pretty much every time they touch a used pistol. You could buy another recoil spring assembly. If that spring changes nothing, then keep it as a spare. You'll be needing it soon enough anyways.
When you notice recoil sharpening, replace the spring.
Also check that there isn't any thick lube in the striker channel if you want to be be that careful.
The stinging finger is your stock trigger safety kicking you when the Glock resets. You can flush cut the safety to mitigate that. Use a G34.3 trigger, hold the safety in from the rear, Dremel the safety lever face flush with the trigger face. Don't hit the trigger shoe though. And finish sand with 2000 grit. Careful, or you'll have to do it again.
