Blender Git Loki

LibOverride: Parallelize diffing of Main database.

This will not give any noticeable improvements in common editing tasks,
since then usually only a very few IDs are changed and checked for
override updates.

However, it makes full override diffing process several times faster
(happens usually when saving a .blend file, but could also help e.g.
when multi-editing several override objects at the same time...).