Btrfs RAID: Built-In/Native RAID vs. Mdadm
The mdadm-based Btrfs RAID was faster for FIO's sequential writes but for all other RAID levels the native Btrfs RAID was faster.
With FS-Mark for a one thousand file test, the native Btrfs RAID was faster than the mdadm-based RAID.
While running FS-Mark with four threads, the native Btrfs RAID was still faster except in RAID 0 where the mdadm-based setup ran ahead.