Updated 10022022-000728
Quite a few, actually! It places a set of pretty potent limitations on an already very resource-finicky hardware-software pair. I don't think anybody outside Apple knows the full list of things it does (naturally,) but it could potentially at any time mean any combination of things are being throttled on your phone: brightness, data connection speed, wifi connection range, framerate, background processes (which effects pretty much everything one does on a phone nowadays,) push notification retrieval frequency (which can be a good thing, for sure,) and the most basic computer stuff like RAM and CPU speed, to name just a few of the for sure ones.
Especially if a power supply is regularly available to you, I just wouldn't. And honestly, in 2022 I'd suggest only leaving Low Power Mode on when you're not actually using the phone - as in, turn it back off every time you pick the thing up to do anything.