As promised yesterday, DirectX 12 support has arrived for Deus Ex: Mankind Divided. Sadly, it's in beta form only, so results are likely to not be as good as you'd hope off the bat.
To try DX12, right click on the game in Steam, navigate to Properties > Beta, then select dx12_preview and close the window. Be sure to grab the new AMD drivers and the latest NVIDIA drivers for best results.
Reports so far indicate moderate improvement or no improvement in FPS and frametimes for AMD users and worse performance for NVIDIA users. In the latter case, one hopes for an upcoming driver to better the situation.