26% at 1080, which no one buying a $500+ CPU every generation is running. The benefits will be minimal at 2k and 4k, with the same GPU, and a decent processor within the last 5-10 years.
So AMD focuses on it here to say "look, your very last excuse for choosing Intel over AMD is no longer invalid."
Of course I do more "non-gaming" than gaming, so it wasn't very important to me in the first place, and I don't spend enough on a graphics card for this to matter. But I want a lot of cores for fast compilation and great multi-tasking with containers and virtual machines.
Going higher than 1080p, GPU starts to become a bottleneck so you're no longer comparing CPU-to-CPU, but (CPU+GPU)-to-(CPU+GPU).
Testing game performance at 1080p is a well-known and well-accepted way to compare CPU performance.