Very slow (~2KB/s) network file transfer of files bigger than a specific size

I built a small home samba server. It had acceptable network speeds (lower bound for transfer capacity is ~20 MB/s, most likely limited by my WIFI connection).
I can't pinpoint the exact point when it started to become slow, but at the time I did experiments with some rsync configurations where I copied multiple times lots of files to the FreeNAS server (500'000 or more files, 40GB or more).

I played around with this command:

Very slow (~2KB/s) network file transfer of files bigger than a specific size