We've tracked down the problem to a set of notifications that were causing high internal load in our background processor. We've disabled them and we're working on some alternatives to ensure this doesn't happen in the future.
Posted 9 days ago. Jan 08, 2019 - 11:00 AEDT
We've had some reports of intermittent delays with notifications (web hooks, GitHub commit statuses, etc). Our current systems aren't reporting any issues at the moment, but there seems to be a spike in delays every 12 hours or so. We're investigating the issue now.