This incident has been resolved. All queues have been cleared and previews will be created instantly again.
Post Mortem
Due to an undocumented or improper documented change in an update in a package we use for background tasks, the production environment has suffered from a period of inactive tasks over the weekend. We have contacted the supplier to help prevent the issue in the future, we also have taken measures to ensure we are notified earlier on in the process. Our apologies for the inconvenience.