CMS has been designed to run large Web sites. In fact, it is large Web sites that benefit the most from the features of CMS. But this does not mean that CMS site designers can forgot about performance considerations. Most developers could easily write code that would bring any Web site to a crawl. The onus is on the site designers and developers to research performance questions so that they can produce the fastest pages possible.
Performance is not something that is added to a Web site after it is finished. It is an essential part of the site design, development, and deployment. Consider performance at each stage of your project, and you will save yourself an awful lot of effort.