[dunham]

MathJaX is sluggish because it's translating stuff after the page loads (and it takes the target browser + window size into account, so it's hard to move it out of the browser).

KaTeX can work as a preprocessor (but has less coverage of special environments and macros), and looks very promising to me. (And as long as you keep the original text in an attribute or hidden element, you can always migrate to something else later.)

The fidelity of MathML that I've seen has been terrible. The only people I've seen make it work is Apple, and there it's only working for their proprietary iBooks files. They render parts of the equation in SVG on top of the MathML, and they've got a special XML file that's tweaking a bunch of MathML layout parameters. I've tried copy/pasting the MathML into an epub, and it looks completely different, even when viewed in the iBooks app.