since SOAP is over HTTP and using XML
it has a very low performance

currently RMI works over IIOP (CORBA protocol)
so it has same performance  with CORBA
but RMI is java specific, and you can't use it in .net,
so for .net interoperability you have to use CORBA

I don't know about other protocols, if any body can tell us about OSSJ, OPCI, SMI relation to SOAP and CORBA I would be very happy.