I'm an '82 PhD mathematician who has used TeX and LaTeX since near their beginnings, to typeset math papers. There was a long period when TeX was essentially in

I'm an '82 PhD mathematician who has used TeX and LaTeX since near their beginnings, to typeset math papers. There was a long period when TeX was essentially instantaneous, while LaTeX was plodding. Also, if one pried into various LaTeX packages, there was a general tone "User won't do this for themselves or even know we're doing this, so let's also clean up this related situation" that felt like coming home to find a team of house cleaners crawling up your chimney with toothbrushes. Directing mathematicians is like herding wolves; this didn't sit well with everyone. Struggling to agree on TeX vs LaTeX for collaborative projects, I ended up writing a few pages "begin.tex" that at least managed automatic theorem numbering and reference handling, and a few other basics, so we could live without LaTeX. This too was essentially instantaneous, and spread from collaborator to collaborator for a few years until machines got faster. Speed matters: Look at the Vim / Neovim split. Some found the Vim delays acceptable, while others had a vision of the future that required far more sophisticated evolutions to not impose user delays. Vim was forced to catch up.