It’s easy - those are very detailed assets, and more you place them, the worse performance will get. I suggest visiting Polycount Wiki, read articles on game optimization and common practices.
I have a little problem. I’m a beginner developer and I make my first project yet. I made already a lot of things, and I make now the first map. I’m done with the terrain, everything is good (I think). Now I want to place trees, rocks and so on for the forest place and other things later. But I have a problem with it. I placed the trees with the painting section (basic KiteDemo trees) and after a few hundred pieces (that’s not a big number for a forest I think) I become fps drops. How can I solve this problem?
My PC specs:
GPU: GTX 970