I shipped 20MB of XML with a product back in 2014; we loaded it at startup, validated it against the XSD, and the performance for this use case was fine. It was

I shipped 20MB of XML with a product back in 2014; we loaded it at startup, validated it against the XSD, and the performance for this use case was fine. It was big because we did something kinda like what TFA suggests: I designed a declarative XML "DSL" and then wrote a bunch of "code" in it. We had lots of performance problems in that project, but the XML DSL wasn't the cause of any of them; that part was fine. I think "expensive" can mean a lot of different things. It was cheap in terms of development time and the loading/validation time, even on 20MB of XML, was not a problem. Visual Studio ships a tool that generates C# classes from the XSDs which was handy. I just wrote the XSDs and the framework provided the parsing, validation, node classes, and tree construction. This is as "XML proper" as I think it's possible to get. I don't believe that .NET's XML serializer uses any of the open source projects mentioned in your post, so maybe we just have especially good XML support in .NET. I think Java has its own XML serializer, too. I bet most XML generated and consumed in the world is not one of those three open source C/C++ libraries. I think Java alone might be responsible for more than half of it.