Defer Sharing Calculation lets an administrator suspend and resume sharing calculations. This permission affects group membership calculation and sharing rule calculation. Performing a large number of configuration changes can lead to very long sharing rule evaluations or time outs. To avoid these issues, an administrator can suspend these calculations and resume calculations during an organization’s maintenance period.

The Defer Sharing Calculation permission is not enabled by default. To enable Defer Sharing Calculation, please contact Salesforce.

Administrators should plan to suspend/resume sharing calculations during maintenance windows to have minimal impact on users.

For example, you make large number of changes to criteria based sharing rules to allow access to specific groups and territories. Since sharing rule calculation is enabled by default, those sharing rule changes are evaluated immediately and can take a very long time to process and cause your organization to time out.

With Defer Sharing Calculation you can avoid this by suspending sharing rule calculations from evaluating immediately after you make sharing rule changes. You can resume sharing rule calculations during maintenance windows to have minimal impact on users.

Check the below link for more information

https://resources.docs.salesforce.com/200/10/en-us/sfdc/pdf/salesforce_defer_sharing_tipsheet.pdf