An increase in dwell time attenuates the recoil impulse- in other words absorbs some energy that would otherwise be transmitted to the hand as felt recoil. More importantly, increasing dwell time maintains pre-tension on the barrel so that harmonics within the barrel are kept consistent from shot to shot. Trying to run the dwell time too close to the moment of bullet exit is what got Smith into trouble with accuracy in the first place. This is also why the CZ P-07/P-09, Walther PPQ and Sig P320 will run circles around the M&P in head to head accuracy testing.

Actually, Glock and the M&P system of lockup are very different. The M&P in its current iteration uses the curved surface of the takedown lever that bears against the ramped surface on the underside of the barrel to prevent the slide assembly from sliding off of the frame. It is this ramped surface that lifts the barrel vertically under recoil spring force. Herein lies a big part of the problem.
The engineers at Smith did not account for what is known as the modulus of elasticity (Young's modulus) of the slide. Basically, all steels have the ability to stretch and rebound back to the original dimensions unless it is forced beyond its yield point. In the case of the M&P, the slide stretches in the area of the ejection port as the bullet pulls the barrel forward and the case pushes the breechface to the rear (Newton's Third Law of motion).

Consider that the recoil spring drives the slide forward and the breech face contacts the barrel at the rear face of the barrel hood extension. This is the point of contact that pushes the barrel forward. When the gun is in battery, the barrel being pushed forward by the slide (which is driven forward by the recoil spring) cams upward as the ramped surface of the barrel underlug contacts the takedown lever. When the slide stretches, the upward force on the barrel now becomes variable and is dependent on the pressure of the firing cartridge. Unfortunatley successive shots will never provide identical pressure curves (chronograph will show variations in bullet velocity).
In order to maintain consistent mechanical vertical lock up, the barrel bottom lug must bear against the frame's locking block. There are only two barrels currently on the market that provide this. Bar-Sto is one, and ours.

Glock's slide lock does not apply vertical force against the barrel in this manner.