The final version of the documentation for Maya is still under construction. This temporary version is based on the Blender documentation, though its contents is relevant to Maya as well (both variants use the analogical UI).

Heavy UV Maps Handling

Version 3.1.0 of the packer introduced significant performance improvements for handling UV maps with a huge number of faces (a few millions and more). Benchmarks show that the packer is now able to process a UV map containing 15 millions of faces in a few minutes. There are still a few considerations the user should have in mind, when packing such a UV map.

Heuristic search considerations

In order to achieve the most dense packing possible, one should enable the Heuristic Search functionality. Heavy UV maps are no exception from this rule - you should only remember to set the Search Time parameter to a larger value so that the packer has enough time for iterating the algorithm.

One problem with using a heuristic search on a heavy UV map is the fact, that the packer always updates the UV map in the Maya UV editor immediately as soon as it finds a better result. While it is a desired behavior when dealing with a usual UV map, it may be problematic in case of heavy UV maps - triggering a position update for millions of UV faces in Maya always requires noticeable computational and memory resources what slows down the entire process.

Due to this issue, you can disable such immediate UV updates:

select the Add-on Preferences multi panel:
press the Show Expert Options button. The expert options will be displayed,
check the Disable Immediate UV Update option.

With this option enabled, the packer will only report the area, when a better result is found, but the UV map will stay intact in Maya during the entire search. The UV map will be updated with the best result only once, after the search is done.