just another fyi, since i last posted here i upgraded to the latest nss build ( IPQ807X NSS Build ).
i never see these messages anymore nor do i have any pauses etc.
if you are apt enough to compile your own build i highly recommend the nss build by qosimo ( https://github.com/qosmio/openwrt-ipq ).
its very stable and much more performant.
caveat : i use this on slightly higher end ipq807x devices, the 301w and nbg7815.... so i am unsure how this will perform on a lower end device like the ax3600 with 512 ram etc. mine all have 1gb.