There's a lot of variables in that statement even... which modes, which settings, what do you define as "massively", what sort of system with which hardware, etc. Even if this was an area I have looked at (it isn't), that's not as simple (and loaded) a question as you imply.Are you able to shed light on why Fortnite is still massively faster in DX11 even on the newest GPUs? What issues are causing such a huge degradation that Epic has been unable to address?
Doing some quick googling I'm see some benchmarks that seem to show the opposite (and also significantly more stuttering on DX11 ironically), for instance:
Another video from chapter 4 shows them as very similar, on NVIDIA this time:
And another where they trade blows on NVIDIA:
I'm sure you can find benchmarks that show DX11 performing better, but that's kind of the point.