It downloads a lot of JavaScript, it decompresses a lot of JavaScript, it parses a lot of JavaScript, it runs a lot of JavaScript, it creates a gazillion onFoundMyNavel event callbacks which all run JavaScript, it does all manner of uncontrolled DOM-touching while its millions of script fragments do their thing, it xhr’s in response to xhrs in response to DOM content ready events, it throws and swallows untold exceptions, has several dozen slightly unoptimized (but not too terrible) page traversals, … the list goes on and on. The point is this all adds up, and having 15MB of code gives a LOT of opportunity for all this to happen. I used to work on a large site where we would break out the stopwatch and paring knife if the homepage got to more than 200KB of code, because it meant we were getting sloppy.