The game is very laggy and for some reason the screen birghtness is really low when I have it set to high. I use an iPad Air 2.
That is your device telling you to lower the graphics settings. Everything on high on an iPad Air 2 is a bit too much I’m afraid.
One thing iOS does to throttle down is to lower the screen brightness.
The graphics on my iPad Air 2 are set to high and it can even manage to run anti-aliasing, but for the best experience I recommend limiting frame rates or the device gets really hot around the processor. And the low brightness is the device thermal throttling, in order to prevent overheating damage.
I reduced my graphics setting and not only is the game not laggy anymore, but the battery usage seems to have gone down as well. Thanks @schyllberg! You can close this now.