My grandma went through this same thing with her old toaster oven back in the day, swore up and down that preheating was a waste of electricity and she'd just toss stuff in cold. Then one Thanksgiving she tried that with a frozen pie and the crust came out soggy on the bottom while the top got burnt. So she went right back to preheating after that. The thing is though, a toaster oven and an air fryer are kinda different beasts because the air fryer moves heat around so fast. Your test actually makes sense to me, the cold batch finishing sooner but coming out less crispy tracks with how these things work. The heat has to build up in the basket either way, so you're just trading time for texture. I'd say for something like frozen fries or nuggets where you want that crunch, the preheat earns its keep, but for leftovers you're just warming up, skip it and save the minutes.