Thank you Jorge. I just realisied that the 18TB is not connected via the card, but directly to the MB. So it seems that the card itself is a bottleneck, although it should have enough wiggly room with 6GBs to easily max out all of the harddrives currently attached to it. As the 4TB and 3TB HDD are only spinning at5400 RPM, I think it might be a combination of not so young HDD´s and the need to move data across the bus from the motherboard to the card.
Lets see how much of a day-to-day impact this will have. If the problem becomes a daily issue, I will likely switch the SATA controller card to one that has been recommended in the forums. If this happens, I will report back with my experiences.
[0:0:0:0] disk SanDisk Cruzer Fit 1.27 /dev/sda
[1:0:0:0] disk ATA ST18000NM000J-2T SN01 /dev/sdb
[2:0:0:0] disk ATA SanDisk SDSSDH3 20RL /dev/sdc
[3:0:0:0] disk ATA ST18000NM000J-2T SN01 /dev/sdd
[4:0:0:0] disk ATA ST18000NM000J-2T SN01 /dev/sde
[5:0:0:0] disk ATA ST12000VN0008-2P SC61 /dev/sdf
[6:0:0:0] disk ATA ST12000VN0008-2P SC61 /dev/sdg
[12:0:0:0] disk ATA WDC WD40EFRX-68N 0A82 /dev/sdh
[36:0:0:0] disk ATA WDC WD30EFRX-68E 0A80 /dev/sdi
[37:0:0:0] disk ATA WDC WD40EFRX-68N 0A82 /dev/sdj