why does unity lag?

my question is why does unity lag when i add grass and or trees or even when i add animations or textures i noticed that the frame-rate decreases is there a way to achieve greater performance while having a decent amount of vegetation like trees and grass, or what technique should i use while creating a new map to reduce the decrease of frame rate.

It must be because of the number of objects on screen, which require more recources. Try reducing the amount and/or size of grass/trees, or lower the graphic quality one or two levels. It should increase the performance.