Webservices( SOAP over HTTP/HTTPs) is not as efficient as RMI/IIOP for multiple reasons: SOAP call stack implementation, XML element parsing, Text transformation etc. What I found is your data structure/content has big impact on performance. On the other hand, a good webservice platform is very important, if you rely on earlier version of AXIS or Castor kind of stuff to handle xml to/from object mapping, you definitely will run into memory and speed problem, but today there are many good products implemented in Java. I don't know .NET side, but I believe things will get better everyday.
I have implemented a jbuilder WebService plugin that can access EJB server object as easy as accessing RDBMS, I am looking for a volunteer to copy the idea into VB.NET, if you interest, let me know.