This has been a topic for years already, but even after the already few months old DX 11 boosts, Fortnite Performance mode (rendering mode) is very bad. DX 12 runs perfectly fine but has massively higher input lag and certain disadvantages compared to Performance mode.
GPU utilization is extremely low and FPS dips whenever a slightly more demanding frame is shown. I'm quite sure this has something to do with shader caching and how inefficient it is.
This has nothing to do with how powerful the GPU is, because even on RX 6900 XT the game can't get more than 150 fps in certain scenes on the rendering mode made for GPUs that are 5-10 years old.
This is an AMD driver issue and doesn't happen on any other brand GPUs.
I hope this gets fixed ASAP because it's unbearable how bad the performance is for such a powerful system (R7 5800X, 6900XT)
Im sure other people have the same exact problem so it's necessary we get the attention to devs so they can look into it.