Huh okay that's interesting - not at all the issue I thought that was happening at all if there's a difference inside and outside of prefabs.
It could be something with a newer version of Unity forcing some editor code to be called more than it should (There shouldn't be a difference at all when editing the color value of a mesh inside and outside of a prefab), so will have to look into that for sure. I'm getting a few ideas already but will need to test them myself to see if they make a difference and don't break anything else. (Marking the textMesh variable as nonserialized is the first thing I'm going to try, but it should not be taking 15-60 seconds to save a 4k vert mesh on any CPU from the last decade regardless...?)
One guess I have that you can try is... is auto-save enabled in the prefab editor? Not sure if it would make a huge difference if it's somehow taking 15-60 seconds at any point, but that would force a save every time a value updates and force other instances to update too which could be a lot if done every single frame.
So will experiment with this soon! Sorry for the trouble.