Against better opponents, nair doubleslap can work couple of times if you time the second slap when they try to grab/jump out of shield and then immediately do something else like block their attack, full jump nair ect. cause they are bound to cc that second slap or just jump right out of it. Anyways, more experienced opponents, even if slapped when they do something outta shield are gonna do immediately something else, like aforementioned cc to something like grab (goo falcon) or jump into stuff (fox dair gg). Personally I'm trying to shy away from using slap alltogether, the best way to manage peach matchup especially for fast fallers is being jumpy cause that way you can avoid about all her ground based attacks and close ground floathed aerials, thus lotsa opponents will jump to make you miss quite a lot and one slap makes you vulnerable to that jumping. Slap is also so very easy to cc it's not funny, even from shield, basically making you a sitting duck to whatever they wanna pull off with that lousy hitstun. Fair to slap to dsmash can work sometimes though if your opponents expects to ger a grab outta you.
Doubleslapping is good when you need a quick interruption and you're just in range, one slap is good if you predict opponent and counter/move out of the way in time. For pressure, slap really needs to be accompanied with something nastier with more histun in order to make your opponent keep to their shields instead of always countering that one slap and if you're really techy and quick on your feet, you can make it work. For example, spaced fair to slap to dtilt iasa to more slaps, or fair to slap to nair to dsmash and stuff (use dashed nair aka forward moving to move inside shield, so dsmash deals more hits).