What kind of oven is underpowered? Most ovens for sale in the EU go upto 250C-300C. Sure, it takes 15 minutes to reach the max temperature, but that's to reduce energy use and the ovens I've seen have a setting for fast preheating.
I know nothing of the technologies involved but I suspect the air fryer makes things easier because it's easier to get the desired effects in a small heating area (the basket) than the oven.
This means it’s under powered. On top of that, the temperature might be high but energy delivery is not. That means the temperature right around the items actually drops, like how being still in cold water creates a static layer of warm water that keeps you warmer than if you’re moving around in it. With fast moving air you get significantly more heat transfer.
This is what convection ovens do but those are under powered in both heat and air flow, for my use case. Enough that I don’t use them at all, while I use my air fryer multiple times daily. Mine actually has the form factor of a toaster oven, so it’s a very easy comparison.
It’s literally just a stronger version of a convection oven, with different marketing.
> reduce energy use
I’m not so sure. It takes my air fryer much less time to cook things, so without doing the numbers I can’t say which is more efficient. I’d say that’s more about cost.
An oven that takes longer to get to temperature is wasting energy, not using less, because of greater waste from losses to the surrounding environment.