In order to improve the performance of a IIS hosted WCF service, you may need to adjust the application pool settings. The following code snippet shows an example configuration setting:
<system.web>
<applicationPool maxConcurrentRequestsPerCPU="x" maxConcurrentThreadsPerCPU="x" requestQueueLimit="x"/>
</system.web>
Refer to the following MSDN page for additional information:
http://msdn.microsoft.com/en-us/library/ee377050.aspx