It depends on the format of the text, the parser for that format, the language of the parser, and many other factors. Such a broad affirmative statement that text files are inefficient is tenuous at best.

Then pad fields and require them, even if the value is Null/None/Nil, and the field names too.

  USERNAME:dotancohen\0\0\0\0\0\0
  FOOBAR\0\0:\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0

This might look ugly if you open it in Notepad, but so do XML and Org mode files. However an Emacs/VIM/VsCode plugin could make this seamless for the user.

We need some technology that turns text files into binary. To the startupmobile!