In this case it was a missing index, on a query run on every order line when saving aka a lot. It had gone under the radar because the volume with our other cu

In this case it was a missing index, on a query run on every order line when saving aka a lot. It had gone under the radar because the volume with our other customers had been low enough that the table scan of the queried table wasn't noticed. As mentioned a customer suddenly got 1000x the volume and it quickly became an issue. But yea, we have a job running in the weekends to recalculate statistics on key tables, as we've had issues with that grinding production to a halt before. And recently I sped up a query by 100x by removing a filter from the where clause that for some weird reason caused the DB to run a really poor plan. It was just a simple check intended to filter out some duplicates in a few edge cases, but couldn't find a way to make the DB run it as a post-predicate. Moved it to my code for the 100x performance win...