Now, I am probably wrong but am asking this due to curiosity. Could they conceivably use HTML5 to render dynamically thus reducing resource/load cost?

Edit: No to Now

I hope they don't try to sue me for this post.

    http://www.wolframalpha.com/input/?i=2+%2B+5

(oh noes!) gives an image containing '7'. They could just freaking put a 7.

Granted, not all of their queries could be done this way, and some of those could indeed be done with canvas, for example. Dunno if those would be faster. But all of the numbers and tables could stand to just be numbers and tables.

This is more of a gripe about copy/paste than performance, really: I'd imagine their calculations take a lot longer than making a .png

Makes sense.

What if they output in LaTeX for more complex things, and there was a LaTeX plugin for browsers?

I've seen Chrome plugins and I know there are Greasemonkey scripts for automatically LaTeXing plain text on web pages.

I don't think this is a viable solution for W|A, however.

MathJax for the win :)

Maybe it's a guard against common scraping?

The rendering is not the hard part - the mathematical calculation is.