Merging charts is too slow

[DebaoHuang]

Hi, for my test of a mesh with about 5M faces, it takes about 41 hours to generate the UV maps if enabling the merging charts option and only 1.5 hours if not. The number of charts will reduce from 33182 to 27362. Could you give me some hints that which parameters could I adjust in the merging charts function so it could take less time while still be enabled?

Also, since my test model is a photogrammetry model (area with some buildings), I tried to tune the parameters (based on description in the header files) so the faces in the same plane (e.g., on the roof or facades) could be in the same group. However, it seems not change so much for the results. Could you also give me ideas about how to tune the parameters for this specific case?

Thanks for your time. I'd appreciate your help!