I'm interested in this -- very similar problem and have confirmed with Windows & Ubuntu clients -- also using ZFS.
Don't assume ZFS is the problem -- have confirmed ZFS speeds.
My config: Unraid w/ZFS on 20 core Xeon server w/ 128GB RAM. 16x6TB NAS Drives in RAID10 configuration (pool of mirrors), Optane 905p SLOG, NVME L2ARC.
Local testing: With this configuration, I'm getting multi-GB/s reads and writes with local testing (as expected -- 'fio' and 'dd')
Network testing: Iperf3 (server on Unraid box, client on various machines) -- all 9+GBE iperf3 results send/receive. Confirmed on Windows10 and Ubuntu client
Real testing using SMB w/10GBE clients:
- Windows 10: using RAMdisk copy large movie files to Unraid ZFS Samba share: write speed 1.1GB/s (perfect)
-- : copy same movies back to Windows 10 RAMdisk ... @ ~500 MB/s peak
- Ubuntu 20.04: using RAMdisk copy large movie files to Unraid ZFS Samba share: write speed 1.1GB/s
-- : copy same movies back to Ubuntu RAMdisk ... @ ~500 MB/s peak
Something is limiting my read-speeds from the Unraid server over SMB.