IPSC is not big on slide-lock reloads. I've not shot IPSC but have several friends that do.
They generally reload whenever they want, and leave mags with rounds in them behind, if it helps speed up the completion of the stage.
If you count rounds, you can shoot your gun until there's a round in the chamber and the slide is shut. If you then drop the empty mag and insert a new one, and continue, you've done a speed reload.
It would be fractionally faster, since you'd not have to release the slide.
IDPA doesn't allow it.