The PreTeXt Guide

The -r (--restrict) switch deserves special mention. It is followed by the value of an @xml:id attribute present in your source XML file. Then whatever action the script is asked to perform, it will only act on a subtree of the hierarchy, rooted at the element with the given @xml:id value.

So if your images are complex or numerous (or both!) and take a long time to process, you can restrict attention to whatever part of the document you are actively editing, and you can even restrict to a single <image> and so produce just a single graphics file.