At the time of writing (v 0.11.0.51 // Nov 13, 2020), the official release of Special K is incompatible with this game (DRM / D3D12 / DXGI Debug issues).
A pre-configured standalone release of Special K (v 0.11.1) is available for this game.
Download SK_ACV.7z (7.6 MB), and extract it to your game’s directory (usually C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\games\Assassin’s Creed Valhalla)
The configuration that ships with this release is tuned for 60 FPS, if you want a different rate the only change you need to make to dxgi.ini is TargetFPS=…
Delete dxgi.dll and dxgi.ini from your game’s directory, no other changes are necessary.
For best results, configure the game to run in Borderless. Waitable SwapChains do not work in Fullscreen, so you won’t be able to use Fullscreen either way and it’s best to not confuse the game when it tries to enter Fullscreen and fails.
I get tired of saying this, but, Borderless is better than Fullscreen when Special K removes all the borderless performance penalties.