This is really useful information, thanks. I'm currently running a 4K external monitor via DisplayPort, using a 2015 MacBook Pro, and having to compromise between either tiny elements at native 3840x2160 and chugging performance at a scaled "1440p" is irritating.Yeah, using it in a scaled mode is pretty much a nightmare scenario for the GPU. The way MacOS is handling this is completely inelegant brute force approach, totally unsuited to a level of GPU power that's available to it. If you want to have a "1080p" desktop on your 4K monitor, that's fine - it renders in the actual 3840x2160 resolution with 2xHiDPI, but if you want to have "1440p" desktop on that 4K monitor, what is actually happening is that instead of still rendering in 3840x2160 but using smaller fonts, icons and window elements, like Windows 10 is doing, Mac OS renders your desktop internally in 5120x2880 and then scales that down to 3840x2160, thus making it both incredibly performance hogging and not as sharp looking, due do to a simple bitmap scaling. That's on top of the fact that even with all the Metal optimizations in the newer MacOS, it's just not very fast when it comes to rendering the UI compared to Windows. Windows 10 is noticeably snappier and has better FPS in any resolution I throw at it on the very same machine, under bootcamp.
I'm due for an upgrade anyway, but I was really hoping the extra GPU grunt in the 16" MBP would be enough to overcome the performance hit imposed by the way macOS scales the desktop. I'm genuinely disappointed if Apple's top-flight "pro" machine can't run their own scaling method outside of perfect 2X situations.
I don't like Windows in many (or maybe any) other areas, but I run a streaming VM on Windows 10 and the simple "scale UI elements to 150%" feature is a far better solution. I can run the desktop at a native 4K but still have good window drawing performance without having to squint to close applications.