By efficiency, I mean it's text and compresses well. If we mean speed, there are extremely fast XML parsers around see this page [0] for state of the art.

For hands-on experience, I used rapidxml for parsing said 3D object files. A 116K XML file is parsed instantly (the rapidxml library's aim is to have speed parity with strlen() on the same file, and they deliver).

Converting the same XML to my own memory model took less than 1ms including creation of classes and interlinking them.

This was on 2010s era hardware (a 3rd generation i7 3770K to be precise).

Verifying the same file against an XSLT would add some milliseconds, not more. Considering the core of the problem might took hours on end torturing memory and CPU, a single 20ms overhead is basically free.

I believe JSON and XML's readability is directly correlated with how the file is designed and written (incl. terminology and how it's formatted), but to be frank, I have seen both good and bad examples on both.

If you can mentally parse HTML, you can mentally parse XML. I tend to learn to parse any markup and programming language mentally so I can simulate them in my mind, but I might be an outlier.

If you're designing a file format based on either for computers only, approaching Perl level regular expressions is not hard.

Oops, forgot the link:

[0]: https://pugixml.org/benchmark.html