>>64034012S&W has been doing a 3.6" barrel M&P2.0 (tiny bit longer than the g26) on a compact frame for a while now. I have one and although it is pants/holster/belt dependent the shorter barrel guns just dig into you less when you sit. Its not a huge difference, but you feel it when sitting. The extra barrel length on a typical compact length isnt getting you a ton. Performance is very similar with typical HP loads. It doesnt conceal any different, just pokes you less. Compact grip length is about as short as reasonable for a lot of people to get a comfortable grip when shooting.
The speed of the slide cycling is a real thing, but actually being able to make use of that is down to the shooter. I dont shoot any faster with a 3.6" barrel vs. 4" or 4.5". From the shooter perspective though, the gun does "settle down" faster after the shot breaks. Yes this is gamer min maxing bullshit. No I do not think it matters.
My question is why the fuck does it have a thumb shelf?