A mobile site is like a backpack — it can be a neat and organized way to carry what you need, so long as you don’t overload it. If you overload your backpack, not only will it weigh you down, but the items in your bag could get wrinkled, bruised or damaged from overcrowding.
Let’s examine the three key factors we’ve found to slow down mobile load times: file size, element order and server requests. The ads, images, videos and measurement technology on a site weigh it down with excess data and requests.
The average mobile web page is 2.5MB in size. This means the data alone takes 13 seconds to download on a fast 3G connection.
Server requests create mobile web traffic jams
When a phone’s browser makes a request to get ads, provide analytics or display content, it is one more task the page has to carry out before a site’s content is fully loaded.
Each mobile page makes an average of 214 server requests, 18 some of which happen simultaneously and some that can only happen one after the other — in any case, that’s 214 potential causes for slow mobile sites.
Source : https://www.doubleclickbygoogle.com/articles/mobile-speed-matters/