WCF naturally produces applications that are both heavily multi-threaded and IO intensive. That makes WCF application performance distinctly influenced at times by the characteristics of the CLR thread pool. The thread pool is a collection of work and a collection of worker threads that it seeks to pair up to minimize the resources and time required to complete the tasks. In the 4.0 framework there aren't as many changes to the IO thread pool as in past releases but there are quite a few changes to the computation thread pool. Eric Eilebrecht has both a blog and a channel 9 video that talks about these.