Sorry, my claim was about the two notations: that the TeX one is writable, and the MathML is not.

I’m not claiming that a JavaScript parser and complete renderer for TeX is better than a JavaScript parser that renders via MathML. The second option may indeed be more efficient - but making the browser parse a sensible notation for maths instead of an XML crapfest would be better than either.

I don't see why rendering TeX formulas would be slower than rendering MathML. It should be possible to maintain implementations for both variants with similar performance. MathML's parser should be simpler though, cause it's XML, which already has many efficient parsers.