Or just use a static site generator? It amazes me how many rarely updated sites use Wordpress when the people who run them are more than capable of using something like jekyl. I guess the themes ecosystem etc around Wordpress is just too big to pass up?
SSGs are good if you think of them in advance. In the short term, such as when one is unexpectedly on the HN front page, switching on caching is the biggest payoff.
A lazier sort of wag might point out that Wordpress runs on MySQL, but that would be a cheap shot unbecoming of our noble selves here at HN.
A more helpful sort might encourage the site owners to turn off the HTTP server, install WP Supercache or W3 Total Cache, then turn it back on.