Nanite Performance is Not Better than Overdraw Focused LODs [TEST RESULTS]. Epic's Documentation is Dangering Optimization.

Speculating here, but thats likely just the rendering thread, in dx12 it probably does operations it cannot conduct in dx11. Think of it more simple, like having athmospheric fog on or off.
If you want more precise, then you should learn how to open up abrender benchmark and compare the 2… it does tell you the timings.