Depending on your engine version there may be an option to tell BSP not to auto build which causes many problems. I think they removed/moved it in 4.3 so not sure on which engine version this existed in. If you turn it off you will have to do your own “Build Geometry” from the viewport Build menu to update your brushes.
It’s talked about in the answer to this post:
“Try turning off update bsp automatically. This is located in Edit>EditorPreferences>Miscellaneous. There is a known bug with this specific feature that is causing slow and freeze issues.”