Yes this is MediaTek MT7621. irqbalance is enabled although I've tried it both ways and it seems similar. It doesn't seem like the cpu governor can be changed on this CPU unless I'm missing it. Shows it is set to "teo".
I tried setting 'adjust_dl_shaper_rate' and 'adjust_ul_shaper_rate' both to 0 but that doesn't change the CPU much. I also tried disabling logging and a small change perhaps but not much.
It looks like the trouble is with the frequency of pingers as if I set "reflector_ping_interval_s=10.0" (just for testing obviously) then the CPU is significantly reduced.
I'm not sure that the CPU utilization of cake-autorate is actually scaling with the download rate but it might just be adding on to the router's own CPU utilization for cake itself which does scale with the download rate. When my network is quiet but pingers are not idle then the CPU utilization on all 4 cores is around 30%. When cake-autorate is stopped it is about 1%. So cake-autorate just doing the pingers takes about 30% and then when I do the download cake itself adds 20-40%, and with process swapping that probably can explain my results.
So then I guess the question should be if I should adjust the ping frequency. My performance is certainly acceptable but it isn't ideal making the router's CPU bump up around it's maximum as that in and of itself could probably introduce latency.