One thing WordPress definitely is not is light. (And it hasn't been a "blogging platform" for several versions now. It's a full-fledged CMS these days. Don't confuse the engine with the themes.) If you have a site that sees any traffic at all, a good caching plugin (something that keeps rendered pages ready for use without having to hammer on the database) is not optional, it's a necessity. And if you're running in an environment that permits it, memcached is a good idea too so that the database hits you do have to make can be proxied in RAM.
All of the popular PHP CMS (Content Management System) software, including WordPress (as well as Drupal and Joomla) get their versatility by making the server do one hell of a pile of work putting a page together. The only thing good that can be said about them is that they're turnkey solutions -- install 'em, pick a theme (or pay to have someone develop one for you), plug in some plugins and you're good to go. A custom solution would take a lot more effort (and cost), but a well-written custom solution can do exactly the same thing using one-tenth (or less) of the resources, so what you pay up front for development, you save quickly in hosting costs (which, for a large and heavily-trafficked site, can run into the thousands of dollars per month).
Now, that may be at the business level, but a hobbyist with more time than money available can realise similar savings. You can easily run a site on free hosting (if reliability isn't an issue) that you would normally have to pay for; you can run a site with an uptime SLA (Service Level Agreement) on low-cost shared hosting that would require a reserved-resource plan (like Infinity+ here at x10) if you use a generic CMS (like Wordpress, Drupal or Joomla).
If you have the time and the inclination, building to suit is always going to win in the long run.
