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
This occurred in 4.12
Head over to the existing AnswerHub thread and let us know what's up.