Which of the following is NOT a feature of SOAP?

The correct answer points out that "Extensively fast" is not a feature inherent to SOAP (Simple Object Access Protocol). SOAP is designed for exchanging structured information in web services, and while it operates over various protocols and supports standards-based messaging, it is often not characterized by speed.

SOAP messages can introduce considerable overhead due to their XML-based format, which can make them slower in comparison to other technologies that may utilize lighter serialization formats. While speed can depend on particular implementations and scenarios, housing such extensible and standard-prone features leads to added complexity and potentially slower performance, especially in high-volume scenarios.

Conversely, SOAP's capability to work over various network protocols (like HTTP, SMTP, TCP, etc.) and its reliance on well-defined standards (like WSDL and XML) are central to its functionality. The data serialization aspect refers to how SOAP transforms data structures into a format suitable for transmission, which is core to its operation. Therefore, the focus on speed does not align with the primary characteristics and trade-offs associated with SOAP, making it an outlier in this context.