I made tree trunks in blender and then used thin instanced leaves. This worked ok for a few trees, but the scene slowed down when I added more than 3 or 4. How best to achieve a forest?
I’d prefer something truly 3d, so not the billboard approach.
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 
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