Sometimes when I play Fortnite on my iPhone X the framerate will get really low, sometimes to the point where it will freeze for a few seconds between frames. Any ideas how to resolve? It didn't do this when I first played.
Thermal throttling is what your phone does when it gets hot playing games. Keeps your phone cool but slows down your game. Obviously closing out of running apps would help.
I'm not sure what you expected when you're playing an online 3D fps game on your phone.
It's not a gaming computer, it's a flappy bird machine.
Of course you can always do what Linus does and watercool your phone to prevent thermal throttling but watercooling your phone is not recommended.
You could also take breaks playing the game so your phone doesn't heat up or keep a cold pack near by.