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 moderat

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.