If you have never tried to ensure your web application will run well then my rule #1 is to not change your application architecture. Program performance is a separate issue that rarely is a problem compared to network latency and server request overhead. I am not saying it is never a problem but you should try things that are much easier first before diving into a restructuring or a rewrite of your app (in most cases buying more hardware is cheaper and safer). As Donald Knuth says, "Premature optimization is the root of all evil (or at least most of it) in programming"
Now that you have done nothing to start (so far so good right?) it is time to do something. Get the YSlow analyzer for Firebug and run it against your web application. It will provide you with a report which you can use to identify possible performance issue. The Firebug network monitor and to a lesser extent the Safari Web Inspector are also good tools for profiling the requests on a page.
Here is a list of a few more performance apps from the RazorSpeed blog and around the web:
- HttpWatch - browser plugin for IE and Firfox for profiling page loads
- PageSpeed - browser plugin for Firefox for profiling page loads
- Hammerhead - an addon for Firebug for profiling page loads
- RockStar Optimizer - an eclipse plugin (update site) for optimizing HTML/CSS/JS/IMG
- Rockstar Profiler - a robust profiler application which uses proxies
Many tuning option are in the hands of your system administrator so if that is not you then you can relax a little bit. However, as a web application developer (frontend/web developer or backend engineer), you should at least know where the common problems lie and this is where the bible of web application performance (Yahoo performance rules) comes in. It is a list of best practices which can be roughly summarized as reduce requests, spread the load, utilize caching and compression, and structure pages for efficiency. If you want the shorter list then check out 14 Rules for Faster-Loading Web Sites (it is just a list of rules taken from the bible with samples). If you prefer an alternative list then try the PageSpeed rules.
If you are lucky you are using an environment that has performance tuning built in (like the grails ui performance plugin or the RockStarApps eclipse/aptana plugin) which will do most of what the performance rules suggest automatically (what can be done in the app anyway). Most web servers provide support for compression so that usually is best handled at that layer anyway. For the rest of the best practices, you will just have to learn and apply the performance rules best practices. In most cases it will be well worth your time.