We just published a great whitepaper written by our WCF/WF Performance PM Saurabh Gupta on the relative performance of WCF compared to ASMX, WSE, Enterprise Services, and Remoting. This is material for your favorites folder. The summary says:

To summarize the results, WCF is 25%—50% faster than ASP.NET Web Services, and approximately 25% faster than .NET Remoting. Comparison with .NET Enterprise Service is load dependant, as in one case WCF is nearly 100% faster but in another scenario it is nearly 25% slower. For WSE 2.0/3.0 implementations, migrating them to WCF will obviously provide the most significant performance gains of almost 4x.

The one scenario where WCF is slower are some comparison scenarios with ES. I'd say that even getting within strinking distance of ES/COM+/DCOM/RPC performance for a V1 release that's based on Web services technology is quite an astonishing accomplishment. The ES/COM+/DCOM/RPC stack underneath had almost 15 years to get to where it's at. And the 4x should give you a really convincing reason to make the move from WSE to WCF.