When I first got Tiger, I thought that Spotlight could replace Quicksilver, which I couldn't live without, but I realised that I couldn't. Spotlight was simply too slow in the face of Quicksilver.
But then I realised that both applications had advantages of their own. Quicksilver's interface was perfect for launching apps and Spotlight was perfect for finding documents and information. So what I did was configure Quicksilver to only search for applications, and told Spotlight to search only for documents, email, etc, but not applications. Spotlight is configured through the Search Results tab of the Spotlight System Preferences panel; just uncheck Applications in the list.
This gave both applications a dramatic boost in performance, but it was especially notable in Spotlight.
[robg adds: This is good advice, regardless of which launcher you may use -- Butler (my launcher of choice) and LaunchBar would probably benefit from similar chagnes. I tested it with Butler (just disabling Spotlight's application searching), and Spotlight did indeed seem a touch quicker.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050716092037920