Mathjax is slow enough that for math-heavy questions or answers my Safari 10 can barely keep up on a 3 years old mac (I am giving these details for precision sake: this is not a bug report but a feature request!). I have been experimenting with KaTeX for my own stuff and I am very impressed with the speed. Their web page offers an interactive sandbox to try it. It has been around for a while and it supports most of what Mathjax does, including matrices, underbrace, a lot of AMS math. The most glaring missing feature is the lack of support for equation numbers (not even \tag is supported) and it might be considered a showstopper for stackexchange. Anyway, I think this is worth a discussion.
So I investigated KaTeX a bit and there is definitively a showstopper for me: there is not support for
and I would find that very annoying as it is a great help to write complex answers.