Maildirs too, though not for so many users, so usually only a few thousand files are transferred, but more frequently some big files of tens of GB each are tran

Maildirs too, though not for so many users, so usually only a few thousand files are transferred, but more frequently some big files of tens of GB each are transferred. The syncings are done most frequently between (Gentoo) Linux using XFS and FreeBSD using UFS, both on NVMe SSDs (Samsung PRO). As I have said, on 2.5 Gb/s Ethernet, the bottleneck is clearly the network link, so rsync, ssh, sshd and the filesystems are faster than this even on old Coffee Lake CPUs or first generation Epyc CPUs. The screen capture from the linked repository of parsync shows extremely slow transfer speeds of a few MB per second, so that seems possible only when there is no local connection between computers but rsync is done over the Internet. In that case the speed is greatly influenced by whatever policies are used by the ISP to control the flow and much less by what your computers do. For a local connection, even an older 1 Gb/s Ethernet should display a constant speed of around 110 MB/s for all files transferred by rsync. When the ISP limits the speed per connection, without limiting the aggregate throughput, then indeed transferring many files in parallel can be a great win. However the ISPs with which I am interacting have never done such a thing for decades and they limit the aggregated bandwidth, so multiple connections do not increase the throughput.