Screen-space reflections are typically rendered at 50% or 25% resolution for obvious performance reasons. Looks like they opted for 25%.

You can mitigate this by downsampling from a higher resolution, though obviously that will hit your framerate that much harder. I personally play at 3.3k horizontal resolution (can't be arsed to look up the exact numbers right now) at a near-solid 60 FPS, but then, I'm running two GTX 970s, so YMMV.