From what I've gathered, the port was handled by the Russian branch of Saber Interactive, the guys responsible for the Switch port, but when that little man with the Napoleon complex sparked a war with Ukraine, CD Projekt Red moved the project in-house."Much worse" was probably an exaggerated statement, but I'd have expected better optimization and performance from a primarily PC-first developer like CDPR.
This was back in April so they had to rush the project out of the door to meet their deadline and it was already delayed beforehand. I suppose they wanted to hit their target to have it released by Christmas but this is clearly a rushed port.
Not sure what they can do with this version of the engine being heavily single-threaded.