How does SMPP compare to XML in terms of complexity to implement and performance?
Do you mean SMPP, the SMS messaging protocol? If so, the difference is that SMPP is a full stack for implementing communications of short messages. It addresses sessions, acknowledgements, error handling etc. as well as data representation.
XML is just a data representation language. It can be used at all levels in a communication stack but "out of the box" as it were, it does not provide anything that compares to SMPP.
SMPP appears to be giving way in the industry to a more loosely coupled combination of layers in the communications stack built around XML for data representation and HTTP/Web services for communication.