I'm not making any assumptions, I'm telling a story about what I watched happen first hand. As you say, you can split up your assets, that does help significantly. You can also put stuff on CDNs, that helps significantly too. Believe me, Twitter engineers are super smart and they've done all of this. But at the end of the day if your business model depends on people looking and interacting with your content, or if you just want your site to be super fast, having a loading spinner while you wait and download the rest of your application in the background isn't going to cut it.