Stored Procedure Performance Issue in Sharepoint Report
Your Sharepoint db instance will have a ReportingService_weirdsequenceofcharacters database. It stores report execution and performance metrics. There is a view called ExecutionLog2 in that database which has 3 Times.
Use them to find whether the bottleneck is sql, report processing, or rendering. You should also be able to compare these values to the values from your production report server (if you have one). A non-sharepoint report server database will typically be named ReportServer.

TimeDataRetrieval

The number of milliseconds spent interacting with data sources and data extensions for all data sets in the main report and all of its subreports. This value includes:


Time spent opening connections to the data source
Time spent reading data rows from the data extension


Note: If a report has multiple data sources/data sets that can be executed in parallel, TimeDataRetrieval contains the duration of the longest DataSet, not the sum of all DataSets durations.  If DataSets are executed sequentially, TimeDataRetrieval contains the sum of all DataSet durations.

TimeProcessing

The number of milliseconds spent in the processing engine for the request.  This value includes:


Report processing bootstrap time 
Tablix processing time (e.g. grouping, sorting, filtering, aggregations, subreport processing), but excludes on-demand expression evaluations (e.g. TextBox.Value, Style.*)
ProcessingScalabilityTime (see AdditionalInfo column)


TimeRendering

The number of milliseconds spent after the Rendering Object Model is exposed to the rendering extension.  This value includes:


Time spent in renderer
Time spent in pagination modules
Time spent in on-demand expression evaluations (e.g. TextBox.Value, Style.*). This is different from prior releases, where TimeProcessing included all expression evaluation. 
PaginationScalabilityTime (see AdditionalInfo column)
RenderingScalabilityTime (see AdditionalInfo column)


http://blogs.msdn.com/b/robertbruckner/archive/2009/01/05/executionlog2-view.aspx