I work helping people set up things like caching. On the three sites I looked at today, caching plugins had an immediate and positive effect.

That serving static files use less resources than firing up PHP should hardly be surprising.

Sure, mod_lsapi and FastCGI are fast, and opcache is a godsend, but static files are still going to be faster than doing multiple MySQL queries, triggering plugins, rendering the HTML, doing whatever post-processing the plugins do, and then serving the result.