Linux

ksmbd is still SMB, except it's implemented within the Linux kernel. As a result, file transfers speeds are improved greatly compared to pure-Samba which runs only in userspace.

The second thing is, you need to check which SMB protocol you're using, ideally you'd want to use at least SMB 3, anything older than that will be painfully slow.

Finally, I read in your other comment that you're using spinning disks and a USB dock. That adds significant overheads.

The Ironwolf drive benchmarks starting at 250MB/s and slows down to 100MB/s as it reaches the end of the drive. (spinning disks gradually become slower the more full it becomes.) Now add file fragmentation + filesystem overheads (buffers, cluster size allocation etc) and the speeds could go down considerably.

Then there's your SATA > USB dock - no dock would ever reach 5Gbps, that's just false advertising - it's only mentioning the theoretical protocol speed. In reality, you'd be seeing something like below 100MB/s write speeds for 128k sequential writes, but if your block size is smaller, expect far slower writes.

Combine all of the above and you can imagine just how much slower this whole thing can be.

For reference, see this benchmark as an example, to see what's "normal" for a simple file transfer to a blank drive with no fragmentation: https://www.anandtech.com/show/6014/startechcom-usb-30-to-sata-ide-hdd-docking-station-review/3