SSRS and Tansact-Sql, shares formatting and manipulation functions, for example calculation, grouping and filtering. For optimization and fast reports you can choose whether this function should be used at Query level or report level. You can choose that query handles the bulk of processing. This will relieve the report from heavy processing and resultantly faster reports. You can also use Report’s calculation, grouping and filtering functions instead of doing these at query level. In this case query will run faster but report will take extra time.

If there are many users accessing the report simultaneously, having report share the processing load also limits the impact on data source server like SQL Server, if the data is residing on SQL server.

There are always different situations which demands different combination of resources.