I am also back on 6.6.7, and the performance is back to what is was prior to 6.7.x.
I have never changed the NCQ setting, and so it is set to 'Auto'.
cat /sys/block/sdX/device/queue_depth = 1 for all spinning disks, but is 31 for the (single) SSD cache drive.
I don't know if this was the case before upgrading to 6.7.x, or if this was set during the upgrade and the new config survived the downgrade. However, as it is set to 1 at the moment, and the performance is fine (with 6.6.7), then I am not sure this is the underlying problem.
Tunable (md_write_method) is set to 'Auto', but i do have the 'CA Auto Turbo Write Mode' plugin installed that turns on Turbo Write when all disks are spinning (and greatly increases my write speed to ~110Mb/s when transferring over SMB). I did not think about turning this off when having issues in 6.7.2, to see if this was a problem or not.
I don't really want to re-upgrade to test things at the moment, until there is some acknowledgement/movement on this issue, as 6.6.7 is working for me.