Root causes found: (on my case)
- The default settings on the current post process volume in version 5.5.3 have added the Hit Lighting option in Ray Lighting Mode, which changes the default value of Ray Lighting Mode.
- The default Lumen settings use “Surface Cache” or “Hit Lighting For Reflection” for Ray Lighting Mode.
- Calculations in the new “Hit Lighting” mode apply to both GI and Reflection, requiring more GPU processing power, resulting in heavier performance impact.
5.4 have no Hit Lighting
5.5.3 adding Hit Lighting option and turned to be default (idk the mechanism)
Maybe this can help u.