I discovered short time ago a simple way to improve performance of certain Reports (RS 2008 & 2008 R2) running in integration with SharePoint 2010.
Imagine the following scenario:
You are using SQL Server Add-in 2008 or 2008 R2 to integrate Reporting Services with SharePoint 2010. You observe that there is a Big different on the time shown in the execution of the report in the ExecutionLog and the real time the report takes from SharePoint.
In the following link you can see the supported combinations of SharePoint and Reporting Services Components:
Supported Combinations of SharePoint and Reporting Services Components
You can observe that:
SQL Server 2012
SQL Server 2008 R2
SQL Server 2008
SQL Server 2012 Add-in can be used for the versions RS Server 2008 and RS 2008 R2 as well as for RS 2012 when integrating with SharePoint 2010.
I suggest you to try using the SQL Server 2012 Reporting Services Add-in for SharePoint 2010 as it includes several improvements and it is compatible with RS 2008 and 2008 R2. You might get surprised and see that some reports might speed up. (Needles to say, always using a test environment first).
You can download it from:
Microsoft® SQL Server® 2012Reporting Services Add-in for Microsoft® SharePoint® Technologies 2010
Reporting Services Support Engineer