Any idea why those two drives are dramatically slower than the rest?

I have eight identical drives connected to an LSI 9207-8i ASRock X470 Taichi Ultimate mainboard.

They're currently all running badblocks -b 8192 -c 2048 -p 0 -s -t 0 -v -w /dev/disk/by-id/ata-WDC_WUH722424ALE6L4_[XXXXXXXX] each.

While sda and sdb achieve almost 300MB/s, sdg and sdh struggle to even get to 200MB/s. That's a 50% difference!

All of my drives are recognized as SATA3, confirmed by smartctl -a /dev/sd[a-h] | grep SATA showing SATA Version is: SATA 3.5, 6.0 Gb/s (current: 6.0 Gb/s)

There's no CPU bottleneck either AFAICT:

Any idea what could be the issue of that? Is such a dramatic speed difference to be expected of Ultrastar drives?Are these drives just bad?

Edit: Mainboard configuration:

Edit2: Okay, apparently the ASM1061 which provides the two extra SATA3 ports is connected to the CPU via a single PCIe 2.0 x1 lane, which provides only 4gbps (=500MB/s) of raw throughput.I think I have found the culprit and the problem should disappear once I connect the drives to my LSI 9207-8i HBA card.