Leap 15.1 uses Sphinx 1.7.6, and Tumbleweed uses Sphinx 2.4.4 (currently), as well as the different Python interpreter versions. I've been working on this along with my other duties, but currently in my testing, I can see the children of sphinx-build are using 1GiB of RAM each, but I'm still investigating. I will look at downgrading Sphinx locally and see what memory profiling turns up.