WCF enables throttling execution of operations but not their completions. This becomes and issue when a large number of outstanding operations complete almost simultaneously causing the callback on the client to be overwhelmed with completions. Generally we don’t expect the client to issue of infinite number of pending operations but if you do end up with very high CPU usage and all suspect all your operations are stuck in the callback method which takes a lock then you need to throttle the callbacks yourself.
You could try Setting the minThreads but this affects the whole app domain. The issue is due to the large number of callbacks that come in concurrently. The sample attached throttles the callbacks to have only one thread execute completions while there are 20 threads starting the operations and all completing almost simultaneously. The idea is to wrap the AsyncResult of your operation and complete only the required number of results in parallel and this would throttle the service operation Ends automatically.