retrieving Large List data

The PortalSiteMapProvider class is one of the best performing List data access methods PortalSiteMapProvider class is going to be most useful if the data you are retrieving is not significantly different over time. If you are trying to frequently retrieve different data sets, the PortalSiteMapProvider class will incur the overhead of constantly reading from the database, inserting data into the cache and then returning it from the method call The advantage of the PortalSiteMapProvider class is when it can read data directly from the cache The amount of memory the PortalSiteMapProvider class has available to use may be somewhat constrained. It uses the site collection object cache to store data; by default, the object cache is only 100 megabytes (MB). for More information on Microsoft performed performance testing against Microsoft® Office SharePoint Server 2007 read this doc 

http://office.microsoft.com/download/afile.aspx?AssetID=AM102377231033