This feature allows users to defer the processing of sharing rules until after new users, rules, and other content have been loaded. This is very useful and handy feature to speed up data loading by avoiding calculation of Sharing rules.
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.