Because, SSRS 2005, it required Internet information services (IIS) to rum, where as in SSRS 2008, it no longer requires IIS. 2008 uses http.sys driver and listens for report requests through http.sys. Not only does this reduce deployment headaches, it also reduces server overhead
SSRS 2005 used more memory and it was extremely resource intensive,so much so that many companies would install it on other machine apart from SQL Server, but 2008 utilizes memory more efficiently,especially when working with reports that contain large sets of data.
Additionally, SSRS 2008 will often load the first page of a report much faster than 2005.
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