移动兼容网站如何处理性能？

As we are now a days fond of bootstrap css which helps in compatiblity. My question is how exactly it deals with the loading as we have more load to the desktop apps and less to mobile. So according to me the css is handling through @media-screen object stylings with toggling of display and alignment. But even if we control the display from client side css or js. Server side will do its task of loading, So whether both desktop website and mobile website are taking the same amount of dataload. As we are now a days fond of bootstrap css whic