With the Sencha dev team hard at work improving the underlying performance of ExtJS 4 there has been a lot of talk on the forums about how to write fast applications. Those of us who've been using ExtJS for a while will all have experienced sluggish UIs and in many cases the problem lay in our applications, not the library itself.
I thought it might be interesting to get some discussion going about what techniques people have used to improve UI performance in their own applications. It would be interesting to compare notes and possibly generate some metrics to see which 'optimizations' actually work. If there's sufficient interest perhaps it could even turn into a blog post.