Why is ZFS slow?

Question

ZFS

ZOL 2.0.3
Debian 10.8
Xeon E3-1265Lv2
16GB DDR3 ECC
RAIDZ1 4x 3TB WD Blue 5400rpm

When writing to the array, the speed can drop to 70-50MB/sec.
When running these disks in mdadm raid10 or raid5 modes, the write speed was 300+.
Moreover, when writing to a raidz array, one io-wait processor thread is loaded. And I don't see a bottleneck in the system. Disks, percent ... not fully loaded. Even when I copy from array to array, the same - simple disks are generally under 50% ... and the speed ... 25MB / sec and lower ((
What should I pay attention to in setting up the system? Or is the situation normal?

Answer 1

What data?
What about fragmentation?

Answer 2

In general, I switched back to mdadm c btrfs .. I left the reliability issue as it is, but created not one raid5 array (as it was), but 3 raid0/1/5 arrays. In an extreme case, the data on RAID1 will still remain, even if one disk out of 4 starts to produce garbage.