Why is my scenes/maps lagging

My maps/scenes starts to lag when i etc add trees, grass or many prefabs (when i am building a map for my fps game) How can i fix this?

You should check your pc for malicous software and trojans…
If not update graphics drivers.

Quite possibly it’s not caused by the software, I’m sure you’ll see the lag won’t be present with other projects or scenes.

Also my version of unity has issues with the video player too, which lags the scene, or when HDR is turned on on the camera component.