Speaking against premature optimization is often misinterpreted as an argument against optimizations. Which it isn't. That's why the word premature is included in the term.
In reality building web apps with what is called HTML5 technology can be quite hard when it comes to have everything perform really well. I tried to put everything I know about optimizing the performance of web apps into one article on html5rocks.com (+ some of what @paul_irish knows about the subject). I hope you enjoy reading it and don't skip over the chapter about profiling :)