One of the missing things still is missing from the main source of WordPress is your typical caching system.
What’s interesting is that this “problem” has been around for ages. Documented from way back in 1.0 somewhere. That’s why plugins like wp-cache or in this case, XCache, exist. The problem with running a WordPress blog without caching is that you run the risk of running some extremely high CPU loads, even with hardly any traffic. That’s not exactly what I call optimized. Don’t believe me? Take it the word from a trusted source along with graphs and all.
Believe me. If you run a WP blog on a host somewhere, your host will thank you for not killing the cpus. Either that or your website will just run really really slow since there might be more than one WP blog on there without caching. Eww.