FPS drop when tier 1 tower down

[fesxpvasya]

I have 150-160 FPS in lobby and when match started i have same 150-160 fps but when tier 1 tower down my fps drop to 45-70 for the rest of the match.
I tried so many way's to fix it but nothing.
upd: FPS does not fall when tier 1 tower falls, but just after 7-10 minutes of the game.