Yesterday I found myself composing a reply to LW in one tab while using 2 other tabs to look up text and URLs so I could paste them into my reply. Under your proposal, MathJax would run in all 3 of the tabs even with Chrome's process-per-tab architecture, and when I needed a page, I needed it now and having to wait for it would have interrupted my train of thought.

How old is your machine? My computer is aging and creaky, and it can easily handle MathJax running in several tabs without any slowdown that I've been able to notice. In particular, any slowdown on page load is so slight that I haven't been able to see it.

There is no upper bound on the memory cost of loading a file of JavaScript, as is true of any Turing-complete programming language.

I was referring to the memory cost of loading the code itself, not of the heap memory which the program may allocate, which obviously will vary from minuscule to infinite depending on the program.

Does this particular JavaScript file not convert LaTex code into images?

No, it doesn't produce images. It generates HTML and CSS, which looks better and is a lot nicer for people wanting to increase the font size.

I notice you have not answered the direct question at the bottom of grandparent.

You mean this one? It must have slipped my mind:

ADDED. Those in favor of MathJax: do you want it for comments or just for top-level posts?

I want it for both comments and top-level posts. Why not?