Recently I had irregular internet access for some weeks and it seemed Zotero was extremely slow. This happened when updating an item, adding a note, or updating a note. It seemd to correlate with the sync icon starting to turn.

I suppose this could be verified with a debug log but before laboriously going offline to generate one I'd like to know first: is it the case that Zotero sync kicks in quite soon after adding or updating items, and that it does so even so when there is no internet connection, and that if there is no connection it slows the whole thing down?

I am talking about delays of multiple seconds, sometimes up to thirty seconds here, which makes it impossible to, say, scribble notes attached to a presentation you're listening to.

By the way, the symptoms are much like those described in this earlier thread of mine, but I haven't experienced them a lot when online, so I thought it could be connected to being offline. My library has only grown larger since them, so if, as Dan suggested, this may have to do with "file scans to detect modified files in [my] (large) library", this is still a problem.