On 2022-09-06 between 10:28 and 12:06 UTC we experienced very slow response times of the Mews application, spiking up to 8s.
We identified a group of regressed queries that were causing high load on database and forced better execution plans for them. There was a full database backup running at that time, which made the mitigation slower.
There was a combination of several factors. One of them being full database backup which runs for a long time due to the database size. It coincided with application deployment, which introduced database changes and contributed to significant regression for a group of SQL queries.
We are going to reschedule the regular deployments so that they do not coincide with database backup.