LOL Lag

Hi guys, I have an issue with lag, when I start a match of League i got 200+ fps, and in the middle of the game i get 100-150 fps, and at the final i barely get 100-120 fps, my drivers are up-to-date. Pc specs: 16GB ram 2400 mhz(dual channel) CPU: Ryzen 5 2400g 3,9 ghz GPU: RX 580 Gaming X MSI 8GB