It isn’t that limited, but for zfs arm seems to perform much worse. Plus you often don’t get a full idea of true system load. the biggest limitation is the io, it is very bad for 5 drives in zfs raidz1. the data is distributed across all 5 drives with parity as well. the pi can only do around 500 meg transfers for an nvme drive whilst many other platforms will see 3000 meg, that is why it suffers so much in this case as that 500 meg is across 5 drives. tops you’d get is 10 meg transfer I reckon and that is roughly what you are seeing. you’d be better off with 3 larger drives in raidz1