bug? dsync extremely much slower on forward than reverse direction

I have been experimenting with dsync lately, and to my surprise I found that dsync in forward direction is extremely slow compared for reverse (-R) direction for the exact same data, disk caches flushed both times. I have created a docker-compose setup to prove my findings. Am I missing something here, or is this a bug?

link to tests: https://github.com/bjne/dovecot-dsync-benchmark

-- Bj(/)rnar