/etc/cron.daily/inn-cron-expire in inn-2.2.2-7 doesn't specify "expireover" on the news.daily command line, so overview database files are never expired, i.e., they list every single article that ever passed through the server, even articles that have long since expired. Hence, nnrpd has lots of delays while it parses huge overview files with lots of useless data in them.
My version of INN (inn-2.2.2-3) documents the 'delayrm' option as implicitly specifying 'expireover'; therefore the supplied inn-cron-expire *does* call expireover. Read news.daily(8) - I may well be wrong about later versions.
You will see from reading the code in news.daily that the "delayrm" option does not, in fact, cause expireover to be invoked. This means that either the man page is correct and there's a bug in news.daily, or news.daily is correct and then man page is wrong. In any case, this bug stands :-).
The current inn package shouldn't create any overview files so they also don't have to be cleaned up. :-) I will enable that in "newsfeeds" and will then also change the cron job to expire overview files in the near future.