WordPress is usually praised for usability, not performance. To make WordPress faster and more efficient, you need a caching plugin like W3 Total Cache. It caches your pages in static files, caches queries and results in memcache, and rewrites css, images, and js to CDN paths if you have a provider. In front of this, I would have Varnish cache as much as possible in memory, for even faster access. W3 Total cache can even invalidate your Varnish cache when pages are updated.

Cannot give Varnish enough praise, it can tubo charge even the most expensive / slowest loading pages and uses minimal resources.

I found that a major contributor to page generation times on Wordpress is the time taken to load and parse all the PHP files. I installed a PHP opcode cache (APC) as I recall, and it made a huge difference, because the parsed representation of all the Wordpress code is cached in memory (or disk). I'd do that before installing any Wordpress caching plugins, though total cache has other nice features, like minification. Plus it has three different levels of caching. You can cache entire generated pages, and/or objects and/or DB results.