Maybe your log process CPU is busy writing small batches of data to disk, and as the throughput increases, it writes larger batches to disk, achieving higher throughput per CPU?
That was my suspicion, too.
My other hypothesis is that this might have been another symptom of the “noisy neighbor” problem discussed in Significant changes in CPU load on resolver processes depending on placement in a cluster - #2 by jon. We’ll likely re-run this test with better CPU/memory isolation in the not-too-distant future, and I’ll report back if that makes a difference in this case.