MathJax is definitely slower, but KaTeX never got the developer attention it deserves, and so still lags in support of even some basic LaTeX environments that are used fairly frequently in math typesetting. Generally the order for software is 1) make it work, 2) make it fast, 3) clean it up, so starting with MathJax makes a lot of sense.
My absolute favourite still is https://www.codecogs.com/latex/eqneditor.php, which is super fast, and has a nicely big input box so you can see all the text you've written, with all the convenient buttons for picking that function of which you can't for the life of you remember the exact syntax. WYSIWYG kind of falls flat when there is so much syntax you can't reasonably be expected to remember all of it.