In this paper, we consider the problem of providing QoS guarantees to the execution of applications using the X11 window system. In particular, we offer a system level analysis of the issues encountered when using X11 to serve real-time applications. By using a tracer developed for the purpose we analyse in depth the internal behaviour of the system. The result of the analysis puts on display the adverse effect played by a non real-time scheduler on the performance of time-sensitive applications. Based on this analysis, we propose an alternative solution based on the CBS scheduler and prove its effectiveness by an extensive set of experiments on real hardware.