Polymorphism in XML is when an element is declared to be of a supertype (in the XSD) and its actual occurrences in the documents are of sub-types of this. Here's an example:
An occurrence of such an element will be:
In such cases, not only your XML [de]serialization will be slower when using eBay's SOA Framework (a.k.a. Project Turmeric), but you won't be able to use ProtoBuf.
Assuming you have the similar types LongFieldValue and StringFieldValue, in order to enable ProtoBuf for your service this will have to be converted to an XML Choice like this:
And the element will now look like this: