Thanks for your tool, @jgreco! It helped me pinpoint a bottleneck in my setup – a single connector between the HBA and a 24-port backplane (BPN-SAS3-846EL2). The issue was resolved by connecting two SFF-8643 cables from the SAS3008/9300-8i running firmware 16.00.12.00 to the primary and secondary expander ports on the same backplane. This resolved the "--SLOW--" indication in the parallel array read test upon retesting. For reference, this 24-port backplane has 16x 20TB Seagate Exos X20 ST20000NM007D-3DJ103 SATA 6Gb/s Standard Model FastFormat (512e/4Kn) disks attached to it.