My thinking is the same as yours but I haven’t tested hand-on-holstered-pistol on a timer yet. I know that at closer distances (7y and in) I can generally get a shot from a low ready on a larger target (8.5”x11” paper) in about 0.6s. If I’ve been shooting and dry practicing often leading up to it, I can get it done around 0.5s. I can’t imagine hand-on-holstered-pistol reaching those speeds. From low ready you already have a complete two handed grip established and your arms at the appropriate level of extension. All you have to do is bring the sights up to your eye line and press the trigger. From hand-on-holstered-pistol you have to remove the gun from the holster, orient the muzzle towards the threat, acquire a two handed grip, extend the gun out from your torso area, bring the sights to your eye line, and press the trigger. That’s a lot of steps compared to shooting from a low ready. You can save some time by doing a couple of those steps together (remove the gun from the holster while orienting the muzzle towards the threat; and extend the gun out from your torso area while bringing the sights to your eye and taking the slack out of the trigger) but the same is true of shooting from the low ready (bring the sights up to your eye line while taking the slack out of the trigger). I’ll try it dry first out of curiosity. I might try it live at some point if I can remember.
ETA: I was discussing two handed shooting to keep it apples to apples, but I can see reaching low ready speeds from hand-on-holstered-pistol if you’re close enough to make SHO shots work based on your skill level. Ripping the gun out of the holster and driving it directly to the threat could get some smoking fast times done. Also, we’re talking about differences likely measured in tenths of a second, so do they matter in the real world? Quite possibly not. I think once you’re talking about sub-second time frames, they’re all close enough to not make much of a difference. If I try to hit you in the chest from the low ready in 0.5s while you try to hit me in the chest from hand-on-holstered-pistol in 0.75s, the end result is that we both just got shot on the chest and it probably won’t matter that one bullet impacted a quarter second before the other.