After reports from several users that tested with the new release 1.4.18 I believe the 100% CPU issue along with other related symptoms is fixed. Thanks to everyone who retested!
Here is what happened: the release 1.4.16 did bring a DB schema migration that fixed a design problem that caused "loosing" of comment items in the DB. Comments must be removed when there parent items are removed and this didn't work well before 1.4.16. Now I have to admit I tested the comment removal with 1.4.16 very well and it worked as expected, but I failed to notice that the changed DB schema caused the parent item removal to silently do nothing.
The effect is a slow one: your DB file grows and on each feed merge you get more and more old items that should have been removed due to the cache size setting. Now merging (sometimes including full text comparison) against a growing list of items becomes slower each time. For users with lots of feeds updated regularily Liferea finally became unusable because it was merging items constantly.
Now when you start 1.4.18 you still might see some CPU usage during the first update run, because it has to delete a lot of stale items, but afterwards it should run as fast as earlier versions.
Everyone please upgrade to 1.4.18