Web Services (WS) provide a comprehensive solution for representing, discovering and invoking services in a variety of environments, including SOA (Service Oriented Architectures) and grid computing systems. At the core of WS lie a number of XML-based standards, such as SOAP, that have ensured WS extensibility, transparency, and interoperability. Nonetheless, there is an increasing demand to enhance WS performance, which is severely impaired by XML’s verbosity. SOAP communications produce considerable network traffic, making them unfit for distributed and heterogeneous computing environments such as the Internet. Also, they introduce higher latency and processing delays than other technologies, like Java-RMI and CORBA. WS research has recently focused on SOAP performance enhancement. Many approaches build on the observation that SOAP message exchange usually involves highly similar messages, and exploit similarity evaluation and differential encoding as potential SOAP performance enhancement techniques. Other approaches investigate parallel processing architectures, to further increase the processing rates of SOAP/XML software toolkits. This survey paper provides a concise, yet comprehensive review of the research efforts aimed at SOAP performance enhancement. A unified view of the problem is provided, covering almost every phase of SOAP processing, ranging over message parsing, serialization, de-serialization, compression, multicasting, security evaluation, and data/instruction-level processing.