Are you sure it’s the SFP+? Please read this thread for example.
Bad SMB write/read performance with 4 drives in 2x mirror configuration (RAID10)
Or
Mysterious read speed cap around 300MB/s on both pools (4xNVME stripe, 24xHDD striped mirrors) in beefy server
There are some other threads here in the forum with similar problems (read speed) and it seems it has to do with the drive configuration and how ZFS it handles.
It’s my personal opinion, a lot of other things are possible, including the NIC.