That’s due to the tris count mostly.
You can lessen it a bit with proper LODs too.
Your only significant performance drop with 50 instances would come from having transparency on leaves.
Full trees are generally challenging for the engine to start with. Putting alpha sorting in pre-pass can help if you end up having issues with transparency on 50 instances too.
Since it’s a small number.
I don’t think you are adding leaves though, so it probably doesn’t matter.
Between reducing the initial geometry and LODs you should have a good baseline scene.
Look at the ms counts and the “stat rhi” tris count more than the fps.
You probably want a base cost around 12ms or so for something this simple with instanced foliage trunks…
The more tris on screen, the more dynamic light will cost.