You may be using CSS variables in LESS, SASS , SCSS, Stylus . All these css-processor languages are going to be compiled to css during the build process.
So, your site is actually using CSS and the values you have provided to the variables are hard-coded into the CSS classes like you do normally in CSS .
So don't worry, Using variables can only slow down the build process a little bit if thousands of variables are used but, once build process is complete they are eliminated from the scene and will not effect any performance or anything.