Take a look at the following two documents, the first one is a what's new in vSphere SDK but also have some best practices regarding perf monitoring that is applicable to current VI API http://communities.vmware.com/docs/DOC-9912 and the second one is performance monitoring using the API with some samples and presentations: http://communities.vmware.com/docs/DOC-9840
Depending on how you did your collection, I think best practices is to use property collector spec and using QueryPerf() http://www.vmware.com/support/developer/vc-sdk/visdk25pubs/ReferenceGuide/vim.PerformanceManager.htm... to optimize your queries and only retrieve what you're looking for.
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William Lam
VMware vExpert 2009
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