If you installed the redistributable SQL Server 2008 R2 version of the Analysis Services OLE DB provider on a SharePoint ...

If you installed the redistributable SQL Server 2008 R2 version of the Analysis Services OLE DB provider on a SharePoint server, you must also manually update the registry entries for the Microsoft.AnalysisServices.ChannelTransport.dll file. Failure to update the registry settings results in slow server-to-server communications between Excel Services and PowerPivot System service, and can potentially cause connection failures in certain security configurations.