Joseph Smarr, Chief Platform Architect at Plaxo, Inc., gave this talk at Yahoo! in 2007. In the course of building Plaxo 3.0, which involved an ambitious foray into rich internet application design, Joseph and his team learned a host of lessons about the importance of performance and the means by which it can be achieved. Many of those lessons ("premature optimization is the root of all evil") will be familiar. But some run contrary to what are commonly thought of as best practice in frontend engineering.