[williamstein]

They have a benchmark page, where you can benchmark for yourself: http://jsperf.com/katex-vs-mathjax

[xymostech]

Yep! Also, ignore the IE 8 data on that page, there was a fluke with IE 8's .innerHTML. You can see http://jsperf.com/katex-vs-mathjax/2 for a version that IE 8 supports.