Looks like you have your answer, but there are a crazy number of possible issues.
The biggest cause is misreading the performance specs.
A partial list of other options:
Mechanical drives store data in rings. Outer rings have higher speeds than inner due to constant angular velocity.
Seeks cost a lot of throuput on mechanical drives.
Oversubscribed drive cables.
HBA issues.
PCIe data path conflicts
Slow RAM
RAM full or busy
Extra cpy within RAM
NUMA path issues (of drives are connected to different NUMA nodes. Not an issue on desktops.)
CPU too busy
Transfer software doing extra things
File system doing extra.
RAID doing extra.
NIC on a different NUMA node than HBA (can be good or bad).
NIC sharing the data path in a conflicting way.
There are others. Start with checking theoretical performance from data sheets.
Also, details matter, and I don’t have enough of them to guess.