> you can still vertically scale by adding more disks
Parallelizing IO is a lot different from scaling up CPU power, though. I'd imagine DB server IO performance has a lot less lower-hanging fruit than CPU/software performance.
That depends, the ratio of free bus capacity for data fetch, and free capacity for inter-CPU synchronization is skewed _massively_ in favor of capacity for data fetch. An x86 system is already under-capacity at the cpu/bus interface, which is why we keep throwing more and more cache at the problem and it works.
Similarly in the cloud on AWS fro example, you have publicly available scalability options starting from 5k IOPS up to 2M IOPS, >400x or 3 orders of magnitude. By contrast you're going from 1vcpu to 192 cores, about half the raise, and a lower performance scaling due to the increased cost of cross-package shootdowns.
Yup, they're different, for sure, but the implication that CPU is easier is not all that clear. In either case, with a database style workload, and with either of these engines in practice you're going to hit a limit at the bus in practice long before you hit a limit on compute or disk io, for any sustained workload - bursts are different.
Parallelizing IO is a lot different from scaling up CPU power, though. I'd imagine DB server IO performance has a lot less lower-hanging fruit than CPU/software performance.