It seems they significantly fixed the slowness with the latest Unreal Engine 5.2 compared to 5.1. Previously the slowness put me off, so I was contemplating going back to version 4.x as well. On my low end laptop(Win10, 16GB, NVidia GTX 960M) , FPS was around 20-30 under version 5.1 even after turning off many of the new features(Lumen, Virtual Shadow Maps, etc) . In version 5.2, the editor runs at 60 FPS, and compiled game runs at 70 FPS. I am using the 3rd person shooter template, unmodified at Medium scalability setting with the new features turned off. I haven’t tried a mobile version in 5.2, but in version 5.1, it ran at 120 FPS.
So at least I can now enjoy the easier to develop features in 5.x. Thanks Epic team!