Slow transfer speeds using rsync over ssh to OpenWrt device

I am running OpenWrt on x86. I have a M.2 SSD on the device and am experiencing really slow (40 MB/s) speeds transferring to it using rsync over ssh. By contrast, using the same rsync command to another x86 box (similar hardware) running Arch Linux, I experiencing average speeds of 110+ MB/s.

Any thoughts as to what might be causing the slow down? Could it be dropbear?

For reference, here is an example command:

% rsync -avxuP --delete-after --ignore-errors /foo/bar/ root@

Maybe post a second post repeating your root cause analysis and mark it as solution?

It is definitely dropbear's fault. Using openssh-server, I get speeds >110 MB/s vs around 40 MB/s using dropbear.

I am reluctant to mark as solved just yet... is there a configuration reason in dropbear that would explain the 3x slow down?

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Not that I know of, but I also observe that SSH data transfers to/from OpenWrt devices feels relatively slow...but I never was bothered enough to research this deeper.

Just to confirm this - spent a morning pulling my hair out why iperf3 was showing fast connection; and scp and rync were very slow.

I put dropbear on port x and openssh-server on port y.

scp and rsync over port y is about 4x faster than on port x (all other things being equal)... so it looks very much like dropbear is causing the slowdown and openssh-server is much faster.