As an admin you sometimes make changes in your administration settings to check issues and forget to switch things back off.
You should check your performance overview at regular intervals to make sure you are not making your Moodle server work harder than it has to.
To see the performance overview – go to: Site administration > Reports > Performance overview
Theme designer mode
This mode stops some of Moodle’s cache features while you are working on a theme. This means the server has to re-create the theme cache everytime someone views a page. This can slow down your system by an incredible amount, not to mention causing the user experience to be slower. Disable this if you are not working on theme changes.
You may have changed this setting on your development or staging server when testing plugins, but on production servers, this should be set to NONE: Do not show any errors or warnings.
This is one of those options that you may have enabled on a production server. If you do, then double check that the backup task is running at a time when the server has the least amount of load. Check your statistics and see when is the best time as automated backups can add a sudden load to the system that will slow things down for users.
This is another option that you may also have running. If you do not need statistics to be recorded from Moodle directly, then please turn this off. Again, this slows down your server.
I hope this helps you out and has made you more aware of some of the processes in Moodle that could be causing the server to have unnecessary load placed on it.