ZFS needs a lot of RAM for good performance. Like 10s of GBs. How were your boxes configured?
[1] https://linustechtips.com/main/topic/738402-zfs-memory-requi...
30 drives gets like 600 MB/s on ZFS, compared to 1200 MB/s with Adaptec hardware RAID.
[ps] Also got a lot of bad stuff to say about backuppods. Mostly that they don't sell the correct number of wire harnesses. (They need to sell two but they're only selling one, I assume I'm the only person stupid enough to buy from them...)
You can still use hardware raid with ZFS if you export each disk as its own device and assemble the raid with ZFS. It is not the parity calculations that are slow, it is the additional io operations that are required and ZFS writes these directly to disk. A hardware raid controller writes these io operations to memory. Another advantage with ZFS is if your data compress well, you will get additional speed benefit from that.
I got what I think is a correctly configured ZFS system and its getting the same speeds as most people report online.
NOW the thing is that the ZFS system doing about 2x worse for the dense sequential write workloads compared to the RAID6 HW solution. Not to mention eating a dozen or so GBs of RAM.
Software raid has a larger write penalty, that is why you see slower write performance versus using hardware raid. As I said earlier, writing directly to memory helps. Recommended read: http://rickardnobel.se/raid-5-write-penalty/