Hi,
Have you enabled the option to reduce CPU usage in the background? It can be found from the Main Menu:
Unreal Editor > Preferences… > General > Miscellaneous > Performance > Use Less CPU In Background
That’ll bring CPU usage to around 3-4%, which should be sufficient. The Unreal engine, including the editor, updates in discrete frames using OpenGL to render, so it can’t be fully suspended in the background like a regular, event-driven, cached-rendering OS X/Cocoa application.