Everyone knows that software updates must be performed both on the server side and on the website itself. We have updated our WordPress plug-ins and the WordPress itself in the course of a routine review of our WordPress. Everything worked well, up to the alert from the monitoring system.
Our very strict monitoring has detected a slower operation of the test scenario operating on the website. Tests are based on simulating high traffic on different subpages to catch some problems that are not visible, for example, with a few visitors.
Monitoring showed a much slower operation of the website after the update.
For a single user it was not felt, but for a larger number of visitors, the website definitely slowed down.
Further analysis showed that the traffic between the web server and the server with the database has increased. The number of SELECT queries increased significantly.
More traffic on the database could have meant a problem with the shell on WordPress or the web server.
Wp Super Cache settings
Updating the plugin changed one configuration option from recommended to: “Disable caching for visitors who have a cookie set in their browser“.
This change meant that the cache was disabled for every visitor who had a cookie in his browser. In practice, the cache worked only for people who entered the site for the first time.
Returning to the recommended settings solved the problem.