For a discussion elsewhere, I did some back-of-the-envelope calculations about the relative performance of disks and CPUs since the early 2000s. The number of retired instructions per byte of random disk reads between an early 2000s machine and today has changed by at least four orders of magnitude (possibly six, depending on exactly what you're measuring).

I knew disks had become faster, but that's an enormous change. If you wrote something 25 years ago that was entirely disk-I/O-bound and used 5% of the CPU, running today it wouldn't even be close to 1% of disk bandwidth usage but would be pegging the core at 100%. Even making it multithreaded and getting a perfect linear speedup wouldn't have come close to 1% disk-bandwidth usage.

The delta is less pronounced for large sequential reads.