I replied with a prospective fix to @1billybob1 in the private message.
I suspect the issue is calling Workspace::joinToOutsiders recursively on every part in a massive place with Upcoming Optimization to Collision Detection System - Beta Test on Mac Clients soon enabled (which it became enabled on Servers yesterday).
Essentially with the new Collision Pipeline, welding objects requires a new sub-structure to be created in the engine. Its super optimized if you make all of your welds at once, however calling Workspace::joinToOutsiders one part at a time on a massive level will cause a lot of redundant work that can be resolved by simply collecting the parts and then passing them in a single table into Workspace::joinToOutsiders once.
EDIT: Hmm, based on the code of joinToOutsiders this may not be enough. Investigating.