I believe that CMS are the ideal solution for most of the people who want to have a website. However, they can be very detrimental to it, since a poorly implemented CMS can drastically affect the performance of the site and this negatively impacts the user experience. As a CMS-based website developer, I will not only focus on a clean and professional looking design, but will also pay full attention to every detail of optimization, in order to maintain high performance. I think that the main tools for website developers are to test the speed of the site (GT Metrix, Google PageSpeed Insights or Pingdom), each theme that is tested, file (CSS or JS) that is uploaded or plugin that is installed , you have to check how it affects performance over and over again. Besides this, responsive and SEO friendly is a must nowadays.