SQL Server 2005 Service Pack 3 was recently released and it include performance enhancements for environment running SQL Reporting Services in SharePoint Integrated Mode.
Taken from the "What's New" Section of the SP3 Documentation:
In SharePoint integrated mode, reports typically run slower than the same reports run in native mode. The main cause of this latency can be attributed to SharePoint object model calls that are made. In SQL Server 2005 SP3, the number of SharePoint object model calls that SQL Server 2005 Reporting Services (SSRS) makes are optimized. This optimization reduces some of the latency when you compare report performance with native mode.