Editor windows that contain viewports cause large framerate loss

When editor windows are opened that contain a viewport of some sort, the framerate of the engine drops drastically. Examples include the Blueprint Editor, Material Editor, and Persona. The viewport does not have to be visible either, as this framerate drop does not change when selecting the Event Graph in the Blueprint Editor.

Workaround: Dock any additional editor windows

Regression?: No
This occurred in 4.12

1. Create a new project
2. Run the following console commands to show your framerate and unlock it:
   stat fps
   t.maxfps 600
3. Note your current framerate
4. Open any editor window that contains a viewport, such as the Material Editor, Blueprint Editor, or Persona, and ensure that it is not docked to main editor
   Result: The framerate in the main editor drops significantly, by about 80-100 frames in most cases
   Expected: The framerate loss should be present but it should not be this severe