The reason you are lagging is because you have set AI up in your scene and you used the navigation bounds to capture the playable area. This means any alterations to the landscape means it has to recapture the navigation data. You can fix this by deleting the bounds and only putting them in when your mesh is close to finish.
We’ve ran into this issue before so I’m speaking from first hand experience.
I delete all nav bounds volumes but the problem is still there. But this was a good idea.