Did your Windows update to the new fall creative edition? Microsoft pushed it out to the masses this week and it’s possible that it could have screwed with your game dvr settings or something along those lines. I know there has been all sorts of complaining about the update screwing with game/application performances. Google around for “windows 10 fall creator update fps fix” and you’ll literally see hundreds of posts talking about this.
If it isn’t that, then perhaps you could try disabling the UE setting to reduce usage while in background(can’t remember the setting name off the top of my head). It’s possible that it’s throttling the other other viewports when you are clicked into another window like a blueprint.