I wonder if this is some odd bug introduced from the transform component rewrite they talked about. Really strange to me.
ANSWER: I won’t post as an answer so that hopefully a Unity rep will chime in with a in depth answer of what is happening under the hood. So basically, it appears that my terrain meshes prefabbed parent was in a weird limbo as my only option was to apply changes, and no option to break prefab instance. Once I applied changes, the lag went right away. I also then was able to break the prefab instance. I also noticed that even simply selecting the prefab object in the inspector caused huge lag. So it had nothing to do with the editor script, it was entirely unity prefabs.
I have updated the title of this issue to be more correct to the issue, and removed my initial lengthy editor script issues to make this more straight forward.