While browsing the manual you’re on point. I bet the 2 drives are plug into SATA3_A1 and A2. These 2 are handled by an ASMedia ASM1061
Comment on Any idea why those two drives are dramatically slower than the rest?
CompactFlax@discuss.tchncs.de 3 days ago
Without googling the specs, are those two on a different channel?
Tolstoy@lemmy.world 3 days ago 
jrgd@lemmy.zip 3 days ago Based on OP’s edit, that looks correct. Unless OP checks the topology and can make the claim that drives from the main chipset are the slow ones, secondary chipset drives will tend to perform slower, and the ASMedia ASM1061 has several reports online of poor to trash performance compared to baseline.
Best bet if the full performance is necessary out of every drive would be finding a decent quality PCI-E SATA III riser card that doesn’t use a known flawed driver chip rather use the two ‘extra’ ports on the motherboard.
Lemmchen@feddit.org 3 days ago
Yep, that was the issue.