Some developers use automated timing programs to measure or estimate page download time. But these programs only measure file size, or at best they time HTML transfers from the server. This completely ignores browser-specific and OS-specific performance issues, which are a big part of the wait perceived by the people looking at your site.
Those automated programs that tell you how long your page will take to download over a 14-KB modem, or measure the efficiency of your HTML, are nothing compared to a rigorous human tester. Use a real, live person to time what is actually seen on the screen, because that's what you're really trying to improve. My team keeps charts of both automated measurements and real-world tests. The automated results are almost always wrong about which page performs the best for a human reader. The money I spent on those Synchrosport 910 stopwatches for everyone has more than paid off.
Test Early and Often
I have my teams perform timing tests as soon as the first prototype of a site's pages is ready. There's never a better time to catch a problem in the making than right away. You also want to detect superior performance at the soonest possible moment so it can be leveraged or traded off as the project progresses. My teams also validate their HTML from day one through post-launch. That way, they don't end up reorganizing a mess of tags or nested tables right before launch, affecting performance or causing new bugs.
Speed Up Your Server
Of course, it's not just fast HTML that makes a site race. It's also fast servers and a fast network connection. Most webmonkeys aren't in a position to buy their own network connections and machine rooms, but that's not a problem. Let the experts do it instead: investigate co-location sites both in your area and in other geographic areas to find the one that's the best at serving your content to its intended audience. A good way to do this is by looking at the sites they already host and talking to the people who built and work with those sites.
If you're hosting your own site, make sure your server software is tuned for speed. Dean Gaudet's column on tuning the Apache HTTPD server is an oldie, but still a must-read. Right now, we're still throwing wrenches in our own servers, but we figure we'll be the world experts in a few more years.