What all does it do? I’ve heard it underclocks the CPU or something slow my iPhone will become slow
Just out of curiosity, why would you like to put the latest iPhone with the longest battery life ever in a low power mode?
I’ve had my phone off the charger from 7am to after 10pm with over 7hrs screen on time. And plugged in with 34% battery
Yea I've never noticed any slowdown. Wish you could customize parts of it. I would use it all the time if it didn't lock the lock timer to 30s.It definitely does and you can test it for yourself by running Geekbench while in Low Power mode. You'll get lower scores.
But often it doesn't really matter. Try it for a while & see. You might notice an occasional laggy behavior but it's not aggravating.