Strange slowdown in Sylpheed

I use Sylpheed to keep up with my newsgroups. I like to open the list of messages full page and go through it clicking on the orange envelope icons showing unread posts to make them "read" if I'm not interested in the subject.

Until fairly recently, a few weeks before the AmiCygnix upgrade, clicking would have an immediate effect, but then it suddenly slowed down, so it takes five seconds for the icon to vanish. Thgis has continued since the upgrade.

In order to time the event, I just checked again. The icons in some groups disappear immediately after all. The group where it takes five seconds claims to contain 25,000 posts (all but a half dozen read). Does that have anything to do with the slowness, and can I do anything about it?