Normal for that game, and it is seen with nVidia hardware as well, even with the common 1920x1080 resolution. Also Fallout 4 has a 60FPS cap which must be removed via .ini file editing.
If sustaining 60fps is proving challenging, we have some advice for you: consider faster RAM. We ran through a series of traversal sequences using a Core i5 4690K paired with a Titan X running at 1080p – in short, a scenario designed to remove the GPU as a bottleneck and stress the CPU. We then re-ran the test with faster memory. Below you’ll find our lowest and average frame-rate results from three different configurations:
A recent Digital Foundry video, however, confirms that it might be a driver issue with AMD driver specifically, and as I’m running an AMD CPU, it could be the crux of the issues.