Drupal is a flexible and versatile Content Management System. It has two major disadvantages, though. The first is being written in PHP, the second is storing content in an SQL database.
PHP is an interpreted, not a compiled language, requiring the scripts to be parsed over and over again, each time a page is requested. SQL is a database concept that adds a considerable amount of overhead to information retrieval.
These two facts make up for a substantial performance hit, even on less trafficked websistes.