ye i spent a good few hours looking into this i think its a bad combo of TSR, Shadows and antialias slowing down the gpu, increasing consumption (wattage) and decreasing performance. TSR is def dx12 only optimized according to devs. Im on 4k screen and i think tsr struggles with it. Heres more info.
Theres too many parameters some are poorly documented, maybe a few ui bugs too. For instance i think unreal insights gives some wrong fps timing depending where the threading sleeping happens, inside or outside the render frame, but its just a visual bug.