I did some new tests, and basically it's always the same thing on my Ryzen 3600 with Geforce 1050 2GB. In places where it runs on ultra at 60fps in dx9, it started to be at 45 with dx11. So yes, there was a drop in frames. This shouldn't be a problem on powerful graphics cards that deliver plenty of FPS, but on less robust cards, losing FPS for free is always bad.
I must also reinforce, the FPS seem less stable. There are big drops in frames out of nowhere, and then they rise again.
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In DX9, the game continues to run with higher FPS, and with the camera very smooth when moving.