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That overhead happens about 30% of the time, so an instance uses about 130% cores average. But there are peaks of 200%. When I run 3 instances on 4 cores, they use all cores most of the time, but you see one idle core sometimes (and it´s not uploading).

When running test beds, I run one instance per core (and disable turn skipping), so all cores are used all of the time.

Running a benchmark restricting each instance to a single core would remove that parallel overhead.

If I had to guess... I'd guess that the peaks would be the JVM garbage collection because that does happen in bursts, and does run in it's own thread(s) independent of whatever java code is running.

Can also be the JIT compiler, which compiles code in parallel by default. It activates at least once for each new battle.