so that the process is heavily IO-bound, but also in part CPU-bound.

In that case, your process isn't either of those. It's just regular mixed processing; and in reality as its extracting a very large volume of data, it probably qualifies as memory-bound.

we usually set the queue to a maximum of about 10 processes running in parallel for our 4-CPU server

With a mixed mode process; that is the sensible choice as allows for greater utilisation of both resources.

• Whilst some processes are hogging the CPUs in their cpu-bound sections; other processes can still be making forward process because they have IO completing whilst they are not occupying a cpu.
• And whilst some processes are waiting for IO to complete, there are other processes that can utilise the cpus that would otherwise stand idle waiting for that IO to complete.

But for pure cpu-bound processing, running more processes that there are cpus has the effect of a net increase in overall elapsed time; because it causes more context switches and more cache misses.

In an ideal world of 1 process per cpu, those processes will tend to always occupy the same cpus; thus the caches, especially the L1 caches closest to the cpus, will retain the same data across preemptions. And when preemptions occur; as there are no other processes to be run, the same processes just get another timeslice and pick up right from where they left off.

It's very rarely an ideal world on a modern OS; there are always lots of other systems processes vying for a cpu; but still it is the case that many of those system processes do very little when they get a timeslot -- often just checking one or two flags or ports before relinquishing the rest of their allotment to the next process.