Nixxes tells us that the Insomniac engine heavily utilises occlusion to reduce draw calls, which improves performance. However, when the camera is moving, the renderer lacks occlusion information for details that are suddenly in view, causing a spike in draw calls and a subsequent stuttering effect. Curiously, this situation tends to improve the better your GPU, but right now, limiting performance and not running flat out during city traversal makes the ride smoother overall.