I'd say the most likely reasons for the speed are:
1. Fast server
2. Use of CDN for static content
3. AJAX responses are cached, meaning that pages are served without needing processing time for each request.
Also, the site is quite basic and cleverly put together in terms of it's template code - many others would use double the amount of HTML to produce the same design. Reducing complexity here also increases the speed at which the page is rendered in the browser.