The growing synergy between Web Services and Grid-based technologies will potentially enable profound, dynamic interactions between scientific applications dispersed in geographic, institutional, and conceptual space. Such deep interoperability requires the simplicity, robustness, and extensibility for which SOAP was conceived, thus making it a natural lingua franca. Concomitant with these advantages, however is a degree of inefficiency that may limit the applicability of SOAP to some situations. We investigate the limitations of SOAP for high-performance scientific computing. We analyze the processing of SOAP messages, and identify the issues of each stage. We present a high-performance SOAP implementation and a schema-specific parser based on the results of our investigation. After our SOAP optimizations are implemented, the most significant bottleneck is ASCII/double conversion. Instead of handling this using extensions to SOAP we recommend a multiprotocol approach that uses SOAP to negotiate faster binary protocols between messaging participants.