Forgive the Honeymooners reference. I came across some statistics by Steve Maine that are quite noteworthy.
Now even if you take these numbers with a grain of salt, it looks like WCF is darn quick in comparison to Websphere. Actually its quick in comparison to ASMX web services as well. Something like 30%-50% faster. Nice.
.NET 3.0 hosted on IIS with an Http binding and XML encoding offers 124% better throughput than the fastest WebSphere/EJB Web Service implementation tested; and 46% better throughput than the JDBC (no entity beans) WebSphere implementation tested.
.NET 3.0 self-hosted over Http/XML offers 225% better throughput than the fastest WebSphere/EJB Web Service implementation tested; and 113% better throughput than the JDBC WebSphere implementation tested.
.NET 3.0 with binary encoding over a TCP binding offers 488% better throughput than the fastest WebSphere EJB Web service implementation tested; and 284% better throughput than the JDBC WebSphere implementation tested.