Once you have your application basically working, it's time to improve its performance. You can use Speed Tracer to find out how your application is performing, and you can use a number of tools to address the specific performance problems that you find.
The resources in a deployed GWT application can be roughly categorized into resources to never cache (.nocache.js), to cache forever (.cache.html), and everything else (myapp.css). Client Bundles allow you to move resources from the everything-else category into the cache-forever category.
The Lightweight Metrics system is a tool to find key areas where latency may be noticeable to your end users. It has very little overhead, can report metrics on application load time and RPC calls, you can profile multiple GWT modules at the same time, and can be extended for your own measurement needs. The Debug Panel for GWT uses the Lightweight metrics system. It provides an easy way to collect metrics and test your GWT application.