Lagging while game ios 15 (iPhone 6sp)

Hello everyone, i want to ask for a solution for this. So i just got an iPhone 6s Plus With ios 15.7.4 in it, i use this device for games. i know its an old device but sometimes i play on it. the problem is when i was playing a game like 1 - 5 mins the FPS Are stable (60 FPS) with the max graphics settings, but after 6 - 9 mins the FPS suddenly jumped to 15 - 10 FPS !! even when i change the graphics to the its lowest settings but the fps is still the same, it still doesn't solve the problem, then what's worse after I quit the game, the os becomes slow, becomes unresponsive.. I hope you understand what I mean :)