Sprite packing on large projects is extremely slow. Currently atlases are local-only, since it is not common practice to include the Library folder in source control, which means developers using source control (in our case, git) switching branches are having to re-import and re-pack the atlases often. This is also a problem with CI solutions since they often pull clean clones of the project, forcing the re-import and re-pack.
It seems that because there is no actual asset in the project, that the cache server does not cache atlases. Even though cache server saves us lots of time with the loose assets, we're still spending 15-25 minutes packing atlases that haven't necessarily changed.