First off, apologies if this is the wrong forum to be posting in. Secondly, I acknowledge that this is not a problem with OpenWRT, but an interaction with another product. I just thought it would be helpful for OpenWRT users who may be seeing this issue (and, maybe, someone may know why).
Since late December/early January a number of users of the Plex Media Server product have been reporting high CPU utilisation issues (constant 100% use of one CPU core) on a variety of platforms (Mac, Windows, Linux and Linux-based NASs). Some diagnostic work has tracked this down to an interaction with the miniupnpd process on OpenWRT routers on the users' networks. Specifically, enabling UPnP (but not NAT-PMP) will instantly cause the PMS (Plex Media Server) process to go into high CPU utilisation. This occurs even if PMS is not configured to use UPnP and you've got a manual port forward configured on your router.
There's much more detail over here if you want to know more. Sadly the Plex team member's initial response was that miniupnpd was "not...adequate" so they're blame shifting already, regardless of the fact that it's been fine for years and all previous versions of their product 