I's basically done by reducing the amount of newly downloaded html for each page. CSS is cached if it's in a linked file, and html uses very few attributes.
HTML 4.01 STRICT is a lot more serviceable than XHTML which is only supported by newer browsers. Even then, it can be tough for noob's. For a forum that's modded extensively by people with varying skill levels, I'd go with HTML 4.01 Transitional.
I do think that more CSS text classes are needed to work with the various colors used for backgrounds in differnt areas of the forum pages - e.g. something that shows well on the forum posts page may not in a pop-up and they both use the same text class so you have to make a new text class and change the html in either the posts page or the popup to use the new class. It would be easier if each area where text could be displayed had its own text classes.