Performance is a critical issue in large projects involving the integration of Commercial off-the-shelf (COTS) products. Many COTS-products include data collection facilities for monitoring their performance. But for understanding performance problem in a COTS system, it is necessary to understand the causality of events. This paper presents an iterative process that uses data collection and correlation techniques to locate the source of a performance issue. The Eclipse Test and Performance Tools Platform project (TPTP) supports this process in COTS-based systems using its extensibility mechanisms. An example illustrates how to use the TPTP tools to solve a performance problem in a real situation.