Real-time operating systems generally depend on some form of priority information for making scheduling decisions. Priorities may take the form of small integers or deadline times, for example, and the priorities indicate the preferred order for execution of the jobs. Unfortunately, most systems suffer from some degree of priority inversion where a high priority job must wait for a lower priority job to execute. We consider the nature of the non-preemptible code sections, called critical sections or critical regions, which give rise to this priority inversion in the context of a soft real-time operating system where average response time for different priority classes is the primary performance metric. An analytical model is described which is used to illustrate how critical regions may affect the time-constrained jobs in a multimedia (soft real-time) task set.