> Clock speeds have remained fairly constant and gains are solely in more cores.

Instructions per clock have risen steadily, even though the frequency stays the same. About doubled since 2011 according to cinebench[0].

That being said, I agree the faster machines argument doesn't hold much water.

[0]: https://cpugrade.com/a/i/articles/cbr15-ipc-comparison.png

https://cpugrade.com/articles/cinebench-r15-ipc-comparison-g...