Large monolithic business systems, as for example the business suite Mysap from SAP, are increasingly restructured in system architectures with fine-granular components, which are loosely coupled by Web services. Their intra-system collaboration has usually no need for dynamic discovery with UDDI. Those architectures show many advantages, but cause also a performance problem which might become an inhibitor to their use in operational systems. We experienced that problem with I^3 M, an instant messaging system we developed and present as an example. Measurements with I^3 M show that the performance overhead of Web service invocation among collocated system components may decrease the system performance dramatically. We propose a solution to the performance problem that preserves the benefits of Web service invocations, but provides the performance of "normal" component invocations.