Turn generate lighting off for a moment 
I’m using Unity 2019.2 and while drawing terrain performance became sluggish and I noticed down in the bottom right hand corner that Unity was attempting to auto generate lighting after every draw. I turned it off for now in the Lighting Settings and will re-enable it when the world is more stable.
Another option in 2019 versions is to turn off “Contribute Global Illumination” on the terrain object itself, down near the bottom of its inspector options. It has the same general effect but still lets lighting do its thing on any other meshes or objects you have in the scene if you’re trying to see how their lighting works without using your computer as a space heater.