Today we are entering the public beta phase of MathJax v2.5. This release focused on improving rendering speed and MathML support.
The 2.5 release improves the speed of the HTML-CSS output by 30-40% (depending on content complexity, with higher gains in more complex situations) and introduces a new preview output (CommonHTML) which currently provides a rougher layout but is ~10x faster than the HTML-CSS output; in the long run, the CommomHTML output will reach the quality of the HTML-CSS and SVG outputs.
In terms of MathML support, Content MathML is now fully supported via a new extension and we have improved the experimental support for elementary math elements (with special thanks to contributions from David Carlisle). The 2.5 release also includes over 70 bug fixes to increase the quality and stability of MathJax (see here for details).
If you see any issues please report them here on meta as we are monitoring.