Very slow macsec transfers on OpenPOWER hardware

I'm seeing an extreme performance issue with macsec on OpenPOWER (ppc64le) hardware, both with and without encryption.  The base systems are identical POWER9-based hosts with Intel 10Gbe Ethernet adapters; the base link validates at 9.9Gbps but the macsec link maxes out at around 250Mbps without encryption and only 170Mbps with encryption.

Ordinarily I'd suspect a cipher module isn't loaded, but the results without encryption seem to point somewhere else.  In both cases the softirq load is extremely high (100% pegged on one CPU core).

Any debugging tips are appreciated -- I can't imagine a 100-fold performance reduction over the native link is correct. ;)

Thanks!