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Samba transfer speed from PC -> Server is stuck at 80 MB/s

I have Samba 4.9.5-Debian installed on my server. The server runs Debian 10.

When I transfer a single large file from my Windows PC to the server, I am getting consistent 80 MB/s speeds no matter what I do. I’ve tried adding a few parameters to /etc/samba/smb.conf, from posts on superuser, but nothing worked (even after restarting smbd service or running reload-config).

If I take a speed test on my Debian server, I get a consistent 950 Mbps on both download and upload, which should give me a theoretical maximum transfer speed of ~120 MB/s. 40 less than expected.

I get the same speed on my Windows PC.

Any clues?

EDIT: Hmm, I just tried making a speed test using iperf3 and it seems like my connection speed is limited to about 660 MB/s, which would explain the ~80 MB/s I am getting. Very weird.