You can double the performance of this whole thing by inserting 2 values at a time.

I don't see why you wouldn't just bundle them all into a transaction at that point. The idea of 1 insert at a time is to roughly estimate the overhead/perf in a real environment, where you're only adding 1 entry at a time.

But, for insert, the CPU is not the bottleneck. It's I/O constrained.

Disk I/O has always been the bottleneck for writes.

Counterintuitively, write performance in modern databases is primarily constrained by disk read performance.

That said, it is increasingly rare for disk I/O to be a bottleneck for inserts. When your storage will happily sustain 10+ GB/s of write throughput, you tend to run out of things like network and memory bandwidth first.