One cause may be a slow http proxy. By default, Wcf uses the default http proxy configured in internet explorer. The proxy may be specified directly or in form of a configuration script:
If this is the cause of the slowness you can reproduce it by surfing to any site using a new IE instance - same slowness should occur.
The possible solutions are:
1. Remove the proxy configuration from IE.
2. Configure your WCF binding with UseDefaultProxy=false.