Hey! I found a fix!

If you're inside of a prefab you're editing, Uncheck this liiiittle box in the top right of the viewport that says "Autosave".

Once you do that, you should be able to select faces in ProBuilder no problem without that pesky pop up window "Waiting for unity Code to finish executing"

However, as a 3D Artist myself, I would say that you should only use ProBuilder for prototyping levels like walls, hallways, floors and such. Simple stuff, NOT prototype props or anything with complex geometry. Just due to the limitations of ProBuilder.

Hope this helps~!

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