Although you mention that you are using uci, I think that at some point you are using the luci web interface for sqm to modify and save data.
I say this because there is a problem in luci-sqm that when you save data what you mention about cpu usage happens.
This occurs even in the snapshot-openwrt build and it is not related to nss.
If you don't use luci you shouldn't have the problem you describe, at least I don't.
I don't know how long this problem with luci has been present but I can say that it was "recently" given that a month ago it was working correctly.