How to fix low FPS due to `afterCullingOutputReady.Invoke` phase when using SpeedTree trees with LoDs?

I have a scene with a few hundreds of trees. When profiling rendering performance, I have found out that afterCullingOutputReady.Invoke method during the Culling phase takes majority of the frame time. And the more trees are on the screen the longer it takes (linearly). I have also found out that t$$anonymous$$s time is only caused by trees in the view frustum as trees outside of view do not increase t$$anonymous$$s time.

All trees are clones of 3 prefabs generated by SpeedTree, with original materials, and with 3 LoDs where the last level is a billboard (all generated by SpeedTree). I am using Unity 2019.4.19f1.

Is there any way to cut down on t$$anonymous$$s time? I had to reduce the number of trees in the scene by half to get FPS above 60. Could t$$anonymous$$s be caused by the LoD groups? Occlusion culling is disabled on the main camera.

Here is an output from profiler:

Any help is very appreciated, thanks!