S-GAME is telling Phantom Blade Zero players on Nvidia graphics cards to go back to driver 617.42. In a Steam post signed by creative director Soulframe Liang at 28 Oct, 7:27am PT (29 Oct, 12:27am AEST), the studio says it has "observed some performance instability" with Nvidia's latest 621.01 driver.
It recommends rolling back to 617.42 with the clean installation option and staying on it "until NVIDIA releases its next driver update, which we expect to arrive soon." The same post says the game's day-one patch, about 5 GB, "significantly improves ray tracing and path tracing performance". PC Gamer also reported the advice.
As of 30 Oct, Nvidia's driver lookup still lists 621.01, released on 21 Oct, as the newest Game Ready driver.