In my view performance is one of the – if not the – most important "feature" of any web site. It's something you must have at the back of your head whenever you make a change, but I still find that every now and then you need to step back and look at performance as such. The small "improvements" you make in every release may still add up to damaging the users experience over time. The content itself is rarely an issue, that's most often only 5–20 per cent of the total, it's all the overhead it generates that really matters.
While working on this lately I've come across a very useful tool that I'd like to share. It's a Firefox add-on called YSlow, developed by Yahoo. Once installed (You have to install Firebug first.) it gives you a very good overview of issues that affects performance for the end user. Just go to the page you want to look at, open Firebug, then Yslow, and you see an analysis of all elements on the page.
In one view it displays a long list of action that could improve performance of the site, including advice on moving CSS and JS in the page, elements that would benefit from using a Content delivery network (CDN), components that lack expiry header and page parts that could be gzipped.
The Yslow application also references the Best practices for speeding up your web site from the Yahoo developers network, and I wish I had found it sooner. In one glance it sums up many of the issues we've spent weeks and months figuring out. It also taught me quite a few things I was not aware of...