In my opinion, it is due to the heating up of the device, or it could be due to a software problem. Firstly, when you charge the device, it heats up almost immediatly. This causes the phone to throttle its performance, causing the frame drops. Secondly, sometimes the phone may not turn off its Stamina mode, causing it to continue to throttle performance and FPS. An easy fix, just turn your Stamina mode in the settings OFF and ON again, or just press it a few times.
