I checked again at 5.3.2 and compared with the same project in 5.2.1.
In a nutshell: I think 5.3.x is similar to 5.2.1 (even if FPS are worse by default in the editor).
A bit more detailed:
5.3.2 default editor performance is still a lot worse to 5.2.1 default editor performance. But 5.2.1 is fake. While I compared settings from 5.3.2 with 5.2.1 I tried changing the ResolutionScale (r.ScreenPercentage). While I immediately saw a difference in 5.3.2 (starting from bad FPS and getting better and better) there was no difference at all while moving the slider in 5.2.1 (no matter if I set it to 100% or 30%).
The reson for that was spammed in the console of 5.2.1: “r.Editor.Viewport.OverridePIEScreenPercentage” is set to 1 so it always ignored my resolutionscale changes and kept it to some strange “editor-100%”. Switching this to r.Editor.Viewport.OverridePIEScreenPercentage to 0 in 5.2.1 gives me pretty much the same (bad) FPS as I get in 5.3.2 with 100% resolution scale as well. So for me it seems the 100% ScreenPercentage in 5.2.1 with disabled override differ from the 100% ScreenPercentage with enabled override. Probably some other settings are kept in this mode as well so I’m not 100% sure if something else changes. But I would not run the game finally in the editor anyway so only the non-overriden stats matter.