I don't have experience with the latest versions of MathJax, but in my experience MathJax is slower than jsMath. Originally I was planning on replacing jsMath with MathJax, but decided to keep jsMath and add MathJax for basically two reasons: (1) MathJax is slower and (2) MathJax does not handle delimiters well with WeBWorK's matrix macro (see e.g. Library/Rochester/setLinearAlgebra3Matrices/ur_Ch1_3_1.pg). This last problem is probably more a problem with how the macro is written and someone with enough TeX expertise (John Jones are you reading this?) shuld be able to fix this.
Davide Cervone has worked on speeding up MathJax and probably still is and he may have some suggestions. For one thing with older versions of MathJax Google's Chrome browser is much faster with MathJax than Firefox or MSIE. In fact on Chrome MathJax is only a little slower than jsMath.