For the last few days, danbooru has been working at sub-optimal speed. Although the reason is unknown, I hypothesize that it might be the same reason why the server went down last time. The most likely cause could be thousands of users constantly uploading random and NSFW (pron, lolicon, etc) pictures onto the server. Another suggestion could be users who have concocted their own scraper that downloads every single picture (via filters and such) off the server, thus causing the severe bandwidth shortage. Another possible cause is through mass hotlinking, which can severely anger those who pay for server costs.
Perhaps, danbooru should have limited accounts to only uploaders (via arbitrary means) rather than allowing the whole public able to create one.