How 21:9 (and larger) can contribute to a larger CPU bottleneck

In some games – especially DX11 and older titles, increasing your aspect ratio can result in a higher CPU bottleneck.
Here’s an example. Hitman 2 in DX11 mode at both 1920×1080 and 2560×1440 using the same GPU settings with a GTX 1080 & Ryzen 1600 :

With averages of 71 and 73fps, the results are largely the same – so we know both are CPU bottlenecked.

But what happens when you switch to an ultrawide resolution?

63.3 (2560×1080) and 62.8fps (3440×1440)

All four resolutions are not GPU-bottlenecked, yet the minimums are considerably worse with 21:9. Why? I’m guessing draw calls. Using the same settings in DX12 mode provided 80fps avg at all previously tested resolutions.