Without NSS/ NPU support, ipq8065 is punching above its weight at those WAN speeds - considering that, your figures aren't really surprising (the choppy results won't happen within its throughput bracket, as you can see in the upload direction). I wouldn't recommend ipq8065 beyond ~350-400 MBit/s WAN throughput, yes, you can stretch that with flow-offloading, but the results you're getting are exemplary for this (keep in mind, flow-offloading can only speed up established connections and you're using 16 concurrent threads). Aside from these hardware limitations of ipq806x, you also need to consider that the other end -the speedtest servers you're testing against- are increasingly getting under pressure for real high-speed connections (let's say significantly above 100-200 MBit/s).
While you can certainly give the attached pull request a test, I wouldn't expect it to significantly change the situation either.