Suddenly lower fps, why?

I have the problem that in games such as Fortnite I can manage around 140-180 fps on epic settings in a match, but when I set the settings to the preset low, I get exactly these numbers. Before that it was low around 400-500. It won't have anything to do with "radeon chill" because I'm using an NVIDIA graphics card. But assume that it is something like "radeon chill" only for NVIDIA cards. Graphics drivers are up to date.

It will most likely be because you have activated VSync in these games, i.e. Your graphics card keeps the fps around the refresh rate of your monitor.