Fast, cross-browser, collapsible menu

I need to build an automatically expanding/collapsing vertical menu
system for my company's website. The site is not enormous, containing
only around 100 static pages.

The menu needs to be three levels deep, as demonstrated in this
example:
http://www.stgeorge.com.au/

(actually this one is 4 levels deep)

Another option I am considering is the same kind of menu but only two
levels deep.

The menu has to be supported by as many browsers as possible.

I know there are many different technologies you can use to create a
menu of this kind: CSS, javascript, templates, server-side includes,
etc.

My question is which type of approach is the fastest-loading?

Any suggestions?