Performance was an important issue in the development of X from the initial protocol design and continues to be important in modern application and extension development. That X is network transparent allows us to analyze the behavior of X from a perspective seldom possible in most systems. We passively monitor network packet flow to measure X application and server performance. The network simulation environment, the data capture tool and data analysis tools will be presented. Data from this analysis are used to show the performance impact of the Render extension, the limitations of the LBX extension and help identify specific application and toolkit performance problems. We believe this analysis technique can be usefully applied to other network protocols.