It's the tax paid by an engine when it is transitioned to a DX12 backend for the first time. You can observe this in the Witcher 3 Definition Edition, DX12 cost 30% of performance compared to DX11 with no visual improvements whatsoever. This happens in dozens of games/engines. It is one of the sad realities of PC gaming, and it's long struggle with lower level APIs.DX12 Tax?
Sometimes the DX12 tax affects NVIDIA GPUs more heavily than AMD GPUs, which results in a performance disparity between the two, a good example of this is Borderlands 3, which has a huge DX12 tax (vs DX11), but it affects NVIDIA way more than AMD.
This is the case in Valhalla, which is the first Assassin's Creed title to use DX12, and it favored AMD heavily as a result. Far Cry 6 is the same as well. So was Borderlands 3.
Recent Call of Duty titles can considered a strong case for this as well, as ever since the engine transition to DX12, the disparity between AMD and NVIDIA became huge. This never happened before the transition to DX12. In fact, Starfield can be considered another example of this, as it's the first DX12 title for the studio, but at least they are optimizing it to perform better.
The DX12 tax is also a major reason why some console ports perform better on consoles than their equivalent PC parts. If the said ports come in DX12 flavor, and if the developer doesn't do the necessary optimizations effort to avoid this tax, then these ports perform worse on PC.