Run resolver with higher priority
DNS is a sensitive service and it should take priority over most other services running on the router as it is one of the primary services it provides. The solution is to run it the same way as for example ntpd with lower niceness. I would suggest -5.
From my tests, this solves issues when an update fails because of failed resolving (updater-lists#1 (moved)).
It should be enough to add procd_set_param nice '-5' to the init script in an appropriate location.