But now the game is in a playable, if not perfect, state. The frame rate holds at 50-60 indoors, dropping to 40 outdoors. Some have suggested the game could have been unpacking data in the background, as I ran it just after the download finished. But these problems persisted for a good hour.
Im running a GTX 970, an i7-5820K CPU clocked at 3.30GHz, 16GB of RAM, and playing the game on ultra settings at 1080p with the latest NVIDIA drivers. And, if it makes any difference, I have the game installed on an SSD.