I'm running this device as an AP only and the qdisc for the wlan's are not set (by openwrt) by default. I've set the qdisc manually to fq_codel now (thanks to @quarky - see here) and there may be an improvement. I will also try changing the ATF weights and see if i can't change the netperf behavior now.
Early on i noticed the lack of qdisc on the wlan if's and tried to set them, but I failed. Can't express how happy I am that @quarky noticed/suggested it along with an example.