Why does my iPhone 14 Pro lag at 120 FPS in PUBG despite being capable of 120 FPS?

sometimes i put guided access it helps to make it stable at 120fps sometimes not similarly works with toggling limit the frame rate option i am playing pubg in my iphone 14 pro its runs at 90 fps even its capable of runing it on 120 fps but when i switch it on 120 it starts chopping lagging why? apple there is nothing wrong with game but i thing ios is creating problem wht you guysthink

[Re-Titled by Moderator]