There's a good interview over on the NetBeans Zone with Kirk Pepperdine talking to Geertjan Wielenga. In it he gives insights on the work he does to find performance issues in applications. Rather than diving straight into the code he looks for what he calls the "dominating consumer" through the use of profiling tools.
Kirk will be sharing his performance tips at JavaOne in a talk called Performance Tuning with Cheap Drink and Poor Tools.