Hi ront,
Welcome to our community! 
The scene also becomes slower the more the camera has to render. It does not matter if the camera has to render an actual tree object or “clones”. Rendering is rendering.
To increase the performance of your game, occlusion culling might be interesting:
I also found this thread:
https://forum.unity.com/threads/creating-a-level-with-thousands-of-trees.725318/
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