I put the low IOPS disk in a good USB 3 enclosure, hooked to an on-CPU USB controller. Now things are flipped:
capacity operations bandwidth
pool alloc free read write read write
------------------------------------ ----- ----- ----- ----- ----- -----
storage-volume-backup 12.6T 3.74T 0 563 0 293M
mirror-0 12.6T 3.74T 0 563 0 293M
wwn-0x5000c500e8736faf - - 0 406 0 146M
wwn-0x5000c500e8737337 - - 0 156 0 146M
You might be right about the link problem.
Looking at the B350 diagram, the whole chipset is hooked via PCIe 3.0 x4 link to the CPU. The other pool (the source) is hooked via USB controller on the chipset. The SATA controller is also on the chipset so it also shares the chipset-CPU link. I'm pretty sure I'm also using all the PCIe links the chipset provides for SSDs. So that's 4GB/s total for the whole chipset. Now I'm probably not saturating the whole link, in this particular workload, but perhaps there's might be another related bottleneck.
Lazy but wholesome.