Yep, i have some wd red 3tb drives in a raidz6 pool.

At best I can get 60m/s out of it. Each drive can do about 100m/s sequential but that is rare.

Putting an ssd on for caching read/writes though really changes the calculus of this.

This is... not my experience at all. I have a RAIDZ2 comprised of 6 4TB Seagate drives (the 5900RPM variety) and it can do about 600MBps read/write with moderate CPU usage on an Intel i3. A mirrored zpool of two Samsung 850 EVO SSDs can do nearly 1GBps read/write. That's not a particularly expensive setup.

zfs maybe not, but btrfs does it.