I experienced an issue with Sharepoint Foundation 2010. (Server 2008 and Sharepoint Foundation 2010 64bit)
When I use multiple Reportviewer Web parts on one page, the rendering (and most likely the Query itself) is not executed in parallel, i.e. every
web part waits for its predecessor to finish.
I made two screenshots to show the difference in behavior (both are on the exact same hardware)
Sharepoint 2010 (Total execution Time 32 sec) :
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