SharePoint Object Model Performance Considerations
Andreas Grabner has posted a brilliant article on performance effects of working with the SharePoint object model. He covers which methods you should use over others (like SPContext.Current.List.Items.Count being much slower than SPContext.Current.List.ItemCount), structuring the code, and backs it up with performance stats and details on how to do your own.
I’d say it’s a must-read for all SharePoint developers.