I still slingshot because it's a more sure/reliable action for me, especially since my pistols that are SAO and have a thumb safety that make it harder to clearly reach that slide lock/release. I want to keep that action consistent for every pistol that I shoot, some of which (like my Sig's) have a very different position for the slide lock/release. All my mag release buttons are in the same position, and even some guns like HK's that have ambi and/or paddle releases, I still use my strong thumb for consistency. I am certain that I will lose more time (especially in competition) if I miss the slide release because of extended safety-lever interference or what have you than the time it takes to manually rack back into action. Same thing if I try to use any support hand thumb on the slide release, by strong hand thumb is already in the way.
If I used just one gun/model for both defensive and competition shooting, then yes perhaps I would use the slide release if it was easily accessible. But I shoot various different pistols and like doing so, so again for the sake of consistency I manually rack. And the consistency I feel also helps when it comes to clearing malfunctions.