Editor performance: "CustomEditorAttributes" in Inspector

https://www.dropbox.com/s/nlb58i5h1oymzuz/CustomEditorAttributes.png?dl=0

Whenever I expand or collapse the Transform or Mesh Renderer component, Unity hangs for 0.8 seconds (!) on the function CustomEditorAttributes.FindCustomEditorTypeByType

Does anyone know how to reduce this? It seems like Unity is confused about which Editor to use, but we don't (appear) to have a custom editor for either Transform or Mesh Renderer.