Filebench varmail personality
Mail servers are an integral part of every businesses, and a busy mailserver surely impose a noticeable load on the I/O subsystem. In a similar manner to a DB server, the main problem here is the continuous stream of synchronized writes coming from the SMTP server (BBU RAID card and SSD exists for that precise reason).
ZFS loses some ground here, as thin XFS, but it is nothing compared to BTRFS... the benchmark reports 0 (zero) MB/s. You can argue that the benchmark was flawed, and maybe that it crashed, but the latency measurement shows us something different:
BTRFS was so slow (over 10 sec. latency) that its throughput inevitable fell below the 0.0 MB/s mark (maybe it was 0.04 MB/s or the likes). Basically it was 20 times slower than the others.