Your hardware seems good (really good) for Unreal Engine 5, so I don’t think you should be worrying about the hardware. Try the Use Less CPU when in Background from the Editor Preferences menu. It could be lag caused by the window becoming unfocused.
I suggest some generic troubleshooting:
- Update your graphics card driver: Make sure you have the latest driver for your RTX 4080 graphics card. Outdated drivers can cause performance issues.
- Check your computer’s cooling: Make sure your computer is well-cooled, high temperatures can cause performance issues.
- Check your hardware: Make sure your hardware is functioning correctly, particularly the memory and the hard drive.