Go to:
Edit -> Editor Preferences -> Miscellaneous and un-check the setting named “Monitor Editor Performance”
That’s the first thing I disable when starting a new project. On my system it runs at 3fps when using another program (or while saving) and the editor mistakenly triggers the warning.
For future reference, if it happens again just click the Settings button on the toolbar, go to Engine Scalalbility and set all of the options back to Epic quality. That’s all that happens when you click ok on the warning, it just gets set to low or medium.