Actually it was my bad with that iperf test capped at 10-15 mbits - I’ve backported some patches from upstream kernel and missed some other dependencies.
Anyway there’s really smth strange going on because mt7621 (2x880mhz mips) device outperforms ipq8064 and on par with ipq8065 with NAT/QoS traffic.
If someone wants to test performance difference there is k4.13 stmmac driver in my tree that you can pull.
https://github.com/dissent1/r7800/commit/47751d79795c1d1c67232ef0cd7f24b4a9f3f4c1