It would take much longer to create the NavMesh on the main thread since IIRC, it splits the work off into multiple worker threads to speed up the process. If you have a large level, you don’t want to be using one giant navmesh anyway, it’s way too expensive and resource heavy and if anything your pathfinding will be slower with such a huge mesh to plot through.
There’s probably a delegate that fires when the navmesh has built, you can possibly block the main thread anyway until that delegate fires? You’ll have to do some poking around to find it though, or alternatively dig into engine source yourself. I’m not sure Epic would make this change themselves, since for most games with large worlds generating it in chunks in significant areas is more suitable. Never know though…
Either way (I’m making assumptions about the design of the game here), do you really want players sat doing nothing waiting for NavMesh to build all the time? It’s not very fast and dependent upon the users CPU power, which might not be as great as yours. Some levels take several minutes to build a navmesh.