The speed of KaTeX is great, but the lack of support for diagrams (a la tikz-cd) is what makes KaTeX unsuitable for general adoption by mathematicians (e.g., mathoverflow.net and all online mathematical wikis I know use MathJax). KaTeX has some rudimentary support for diagrams though the {CD} environment, but something more fully fledged akin to tizk-cd or xymatrix is needed. There's been some discussion on their github (https://github.com/KaTeX/KaTeX/issues/219), but I wouldn't hold my breath.
