I think #2053 is supposed to help here, but doitsujin doesn't like it.
Indeed, I searched the issues but not the PRs.
Hm, that PR should be in Proton Experimental still? Is it not?
Looks like it isn't, it's been (incorrectly) dropped between experimental-6.3-20210616 and experimental-6.3-20210625, probably during the 6.3-5 release.
proton experimentalx1 2021-07
The game calls CreateDXGIFactory repeatedly, which can be time consuming as it enumerates the adapters (although it may depend on the hardware or driver).
Proton had a hack in Wine DXGI to keep a single instance of the factory for that game (https://github.com/ValveSoftware/wine/commit/57f8d20c216ddfae0d598b4cb3c93d5e6cb7ea10), and switching to DXVK dxgi introduced a regression.