So after toying around with RAID 0 just for fun, time to get serious. I created a RAID 6 of 10 x 1 TB disks. This gives me raw device read speeds of 850 MB/s and write speeds of 300 MB/s. I think this is exactly what should be expected, but boy it is damn fast. Especially the write speed surprises me.
Still, if I format the device using EXT4, write speed stays the same. However, read speed drop to about 350 MB/s. I don't understand why. Maybe some misalignment? I don't know.
The issue with the dropped read speed is due to an incorrect chunk size of the array. The array now consists of 20 drives, providing 18 TB of storage using XFS. Read speads exceed 1.1 GB/s (that is not a typo) and write speeds are about 350 MB/s.