Submissions are processed asynchronously by a background job. However, this normally only takes a few seconds…
API documentation
As far as i understand normal AcoustID API behavior is put submission to database or some buffer and return status 200 (Ok) to client. Seems like this part of AcoustID work well all the time. But another part, that shall somehow process submission and make it usable for users not work at lest last 9 days