I'd bet a very large amount of money that there is no front-end caching systems in place on xenforo.com... it's simply not necessary. It varies depending on the complexity of the page being rendered (everything... queries, PHP processing, etc.), but for digitalpoint.com simple pages render in about 0.01 seconds... A really complex pages might see 0.15 seconds.
Something like Varnish might actually make things slower simply because of the overhead of needing to check the cache.
Caching is good when you are caching the right things... but I've never been a fan of full page caching... that usually means you have something built poorly to begin with if it needs it.