Conformance testing of communication protocols has recently become a major issue in the context of OSI-based standardization of protocols. The aim of conformance testing is to assure that a protocol fulfils an OSI specification. In this paper a performance study is presented for a distributed protocol test system that has been installed for conformance testing of the ISDN D-channel signalling protocol. Using a general approach for performance measurements and evaluation in distributed systems, a queueing model is developed and evaluated, based on runtimes as obtained from measurements of the test system. It is demonstrated that significant performance improvements can be achieved once the process scheduling strategy at the ISDN protocol testers is properly adjusted.