Xiaomi 13T FPS Drops From 120 to 60 Causing Lag in Games

I am using a Xiaomi 13T phone and for about the last three months I have been experiencing a serious performance problem in FPS games. While playing games such as Standoff and PUBG, the device starts at 120 FPS but after a while the frame rate drops and becomes fixed at 60 FPS, and noticeable stuttering and lag begins. This situation generally occurs in FPS games and disrupts my gaming experience. I bought the phone second‑hand from a user who had used it for about one month before me. Until today I have not contacted any authorized service, store or Xiaomi customer service about this issue, I have only started researching the problem. My only request is that this performance problem on my Xiaomi 13T device be resolved and that I receive support on what steps I should follow. I would like to be informed and assisted regarding the necessary technical examination or possible software solution so that I can use the phone with stable performance in games.