I did a direct comparison between the two with profiler and found out that the performance gains is actually due to TSR quality and Lumen Quality in 5.1/5.2 being the equivalent of Low on High settings. So if you turn these down to Low in scalability settings in 5.4 you’ll see your frames jump right back to where they were. It’s actually a bug that the editor lumen quality settings are not being applied correctly or respected. A few bug trackers that I followed to repo and profile helped shed some light on a few really big hitters.
Unreal Engine Issues and Bug Tracker (UE-225380)
Unreal Engine Issues and Bug Tracker (TM-15457)
5.1
5.5
TSR and Lumen cost are just insane on very very simple scenes in later versions of the engine.