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 this time is only caused by trees in the view frustum as trees outside of view do not increase this 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 this time? I had to reduce the number of trees in the scene by half to get FPS above 60. Could this 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!