I was having the same problem: FPS drops dramatically when the secondary editor window with the blueprint editors and such was open. None of the other solutions had any effect but after some fiddling I stumbled on a fix which was just maximizing the main editor window. It’s a bit hard to notice because the default window size covers the entire screen the same as when maximized even when it’s not. This resolved the issue and delivered full performance even with the secondary editor window visible.
This is under Windows 10, nVidia, dual monitor, UE 4.27
This fix may not work for everyone but try just maximizing the main editor window.