A few hours ago we seem to have lost most of the content of our images Squid caches. While the Squid servers fill up their caches again, image loading will unfortunately be slow and unreliable, because the image server backend is overloaded. The situation is recovering slowly, but it can take a while before the speed is back at a normal level.
This incident was most likely caused by a change I made to an ACL shortly before it started, although as of yet we don't quite understand why, as it seems rather unrelated. Investigation continues...