TechTuesday : Tip 181 - Improve Performance and Stability
In order to improve the performance of ServiceDesk Plus application, you can choose to delete all the system generated notifications for closed and/or archived requests, for which the completed date is greater than 'n' months from today's date. [The value of 'n' can be 3, 6, 12, 18 and 24].
This option is present in Admin -> Self Service Portal Settings.