Horrible FPS in editor with undocked windows, what could be the cause?

I’m experiencing drastic FPS drop as soon I undock any window in the editor. Using the default editor layout lands me on 120FPS. As soon I make say “Place actor” panel undocked and floating, the FPS halves as if it is perceived as a separate engine instance. Undocking content browsing drops FPS further, and so on, with three floating panels dropping FPS below 30:

What could be the cause of this behavior? Already tried removing usual screen overlay/recording suspects as Nvidia GeForce Experience and Discord.

I experience this too. I assume that the engine is drawing any other window per frame and impacts testing. Frame drop also happens if you have a viewport but launch a window to play it as it renders a static viewport while also playing the game. I will keep all my windows docks as you say and switch my viewport to the output log when I test. I’ve gotten use to this over the years and assumed its just engine limitations. Either way, editor frame rate is not dependable even with these fixes. Standalone or a packaged build will produce correct testing performance.