Opening the project fresh, I have 120fps locked on every level, but the moment I open the content browser my framerate tanks to 12fps and I can’t do anything. Closing the content browser doesn’t fix it, but if I restart Unreal and open it fresh again 120fps locked.
Try turning off real-time thumbnails in the content browsers settings and see if that helps.
Is the engine doing any other background work like loading new assets, compiling textures etc while these fps drops occur?
You can also check if you have UDP messaging on in the project settings. It can degrade performance.(try an editor restart after unchecking it)