The brilliance of Drupal as an all-in-one-go CMS becomes evident when we consider its flexible taxonomy structure and modular framework. But something that may go unnoticed is the ability to use it to create multi sites sharing common code base and modules but different databases and themes. Here are some materials that could make it more clear.

Multi-site installation and set-up

Drupal Multi-site for Fun and Profit

After some server side and Drupal cache improvements we have seen around
40%-50% load time reduction in Hotelotravel.com. A good guide into optimizing
Drupal.