This started happening around one week ago. Only the titles will load, the previews won't. If I open the ATOM feed in a text editor, copy the preview links and open them in a browser, they will load just fine. Sometimes I get lucky and 2 out of 100 will show up in the feed reader.
I tried exporting each one of my subscriptions to another feed reader (The Old Reader -> Feedreader) and using proxies, but the problem persists. Feeds from other sites (yande.re and Phoronix) still work. Browsing Danbooru from Firefox is as fast as it's always been.
I think this started happening right after the "Mar 09: New server again, site may experience slowness for next 1-2 days" global message, but I'm not 100% sure.