FPS Problems near the end of every game

When the game starts, my fps is always fine. (I have a decent computer, always getting 150 fps or higher) but near the end of every game i start to get pretty bad fps lag going down to like 40 or 50 fps in their base. This has always been a problem for me since i started in season 5 but never really cared. Can anyone give me some insight about why this occurs? thanks!