Massive increase in lighting build time due to single tree?


I could use feedback on this: working on an exterior scene (static lighting) I added a single tree (static mesh actor, mobility is static, casts no dynamic shadows). This results in the lighting build time to jump up from ~2min to ~4hrs.

  • directional light set to static
  • skylight set to static, using an hdr image
  • lightmass importance volume around main focus area (including tree)
  • no further actors relevant for lighting
  • tree lightmap density 512
  • apart from the tree only 4 more actors in the scene (two planes for the ground and a couple of boxes for further visual reference)

Having baked lighting on much more complex scenes and also containing trees (just other assets) I did not have such long build times.

From other user’s questions I gather that baked lighting on trees can result in longer build times and that dynamic lighting is preferable when using foliage, but I don’t think I should be at the limit of what is sensible using just one tree with lightmass.

What’s wrong with this tree/setup and what would be the most obvious places to start optimizing this?
I have a feeling optimized lightmap UVs could help (i.e. unwrap trunk and branches without each mesh face ending up being a separate island)?

If further info is required I’ll be happy to provide that, just wanted to get this question started.