Do you use thin instances for the trunks as well? How many polygons do you have so that it would slow down at 3/4 
?
Maybe you can share a playground with the tree model for testing ^^
I made a small forest scene here:
The trunks are quite low poly and the leaves are made using quads. That’s not as beautiful as having individual leaves but it’s performant.
If you want to use a high quality asset with individual leaves, you should read about octahedral impostors that are popular for this kind of task (here is a blog post I found very informative on the topic: Post Page).
Although I don’t know if someone implemented octahedral impostors in BabylonJS 