Fallen Feathers developer Leenzee issued an updated statement via Steam, to further apologize for the game's launch issues and promise a "comprehensive explanation" of what went wrong is on the way. In the meantime,

TL;DR: Wuchang: Fallen Feathers faces ongoing controversy over PC performance, with accusations that patch 1.3 boosts frame rates by stealthily using upscaling rather than actual game optimization measures - going by the findings of a YouTuber. While the visual differences may not seem that noticeable, that's not really the point.