Optimizing a Joomla website: a pragmatic guide, part 1

Photo of cheetahs considering optimizing their Joomla site by James Temple used under CC Attribution 2.0. license. Flickr: https://www.flickr.com/photos/jamestemple/312324961/sizes/o/
Introduction
When we build a website on a content management platform like Joomla, Drupal, or WordPress, we are taking advantage of a sophisticated web application and all the built-in functionality it offers. And, we have to accept that as consequence of using these applications we are adding to the functioning complexity of our website. This becomes increasingly so as we add extra functionality through third-party extensions, plug-ins and widgets!
The result is that we might not have complete control of the code that runs the website, including the CSS and JavaScript files installed by those extra extensions.