Some extra findings for today.
I checked how UE4 VS UE5 compare on DX11, and UE5 actually outperforms UE4 (without the translucency 0 command). With this command UE4 shows the same performance as UE5. So it looks like the fps differential is related to how the DX12 RHI was changed in respect to UE4. Also worth to point out DX11 outperforms DX12 atleast in my barebones scene by quite alot (700vs 1000fps). I tried vulkan aswell but its worse.