Hrmm. I have a 2500k and will have to keep an eye on this. I heard the game was really light on cpu stressing.
This is true of the game's rural areas, which is why so many of the initial performance reports about the game said it was light on CPU usage, but dense urban areas are quite the opposite. The game is very well optimized (CPU usage scales across an arbitrary number of cores, and it will use 90%+ of each core under demanding circumstances), but Novigrad is
very dense with CPUs, which I think stresses the game's asset streaming system as you move around the city (CPU usage tends to be highest when moving, as is typical of modern engines).
As a quick comparison, my system typically uses ~50% of each CPU core and 80-90% of my GPU while in rural Velen. In Novigrad however, CPU usage spikes to 90%+ on each core and GPU usage drops off a bit (typically 60-80%), while my frame rate holds around ~50 FPS. The good news is that simply running around the Novigrad doesn't exactly require twitch reflexes, and the game supports triple buffering out of the box, so the stutter is very subtle, easy to overlook.
EDIT: This is without hairworks, by the way, which is known to cause massive performance degradation in any situation with multiple actors near the camera.