How can I speed up zfs raw send/recv of encrypted datasets?

Have you tried without the lzop ? For raw encrypted data, I can’t imagine it would bring too much benefit anyway. Raw encrypted data is theoretically relatively incompressible (not entirely, but it does not perform well under compression).

Also, to be honest, I would advise against using ZFS native encryption right now (unless you really cannot trust the recipient). There are multiple ongoing bugs (data will sometimes be written unencrypted, snapshots can become corrupted, etc.) that put your data at risk (@rincebrain cataloged these on other platforms as well). ZFS over LUKS is the much more standard and battle-tested way of encrypting at rest. You lose the advantage of sending the data encrypted, but the risk of corruption is not worth it, in my opinion (especially if you are in control of the recipient device).