alt Hacker NewsFair, it's shallow because I was succinct however, I do understand the problem space more in depth than this.
But if I were to pick one single thing that would speed up the most UIs across the board, it would be poor handling of the UI in networked systems. As you noted, that doesn't mean eliminating them, it means handling the inevitable in a way that doesn't tank the UI feel.
True, dev environments are fast. One dev implements a wrapper with roundtrips, another integrates it into a UI and no-one stops to think if it'll have terrible lag in practise. They don't notice so even if there's a ticket it'll starve and end up WONTFIX.
Somehow I don't think I'm the only one who presses a button and when nothing happens presses it repeatedly until something happens, or I kill the app, or even power off whatever piece of shit computer I'm using.