At 1080p, with a 2080 Ti (AMD's Testing Rig), the GPU is not a bottleneck so any performance difference = CPU performance difference. Going higher than 1080p,

At 1080p, with a 2080 Ti (AMD's Testing Rig), the GPU is not a bottleneck so any performance difference = CPU performance difference. 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.