Very slow NavMesh baking with multiple terrains

Hey there guys!

Lately I came in a slight obstacle while working on my game. The game world is divided into 12 smaller (500x500) terrain game objects and filled with various buildings, environment stuff etc. Lately navmesh generation takes too long (10-12 hours) making my development and testing process a real pain. I tried testing things out looking what could be the source of problem, and I came to conclusion that if 4 or less terrain objects are active, navmesh generates in matter of seconds, while if there are 5 or more terrain objects baking takes forever.

What could be the possible solution for the problem? What if I create one big terrain (6000x6000), would it be performance and speed wise? Thank you for your time taking to answer my question.

Perhaps setting your terrains to be neighbors would magically fix it?

Usually that just fixes the LOD connections when you move far away from the terrain but maybe it helps the Navmesh baking too.