Open Safari's preferences file at ~/Library -> Preferences -> com.apple.Safari.plist in a text editor and add these two lines somewhere inside the <dict> tag:
<key>WebKitInitialTimedLayoutDelay</key>
<real>0.25</real>
The default value is 1.0. Lowering the value will decrease the time Safari waits to render. Some side effects may be flashing page content as it loads, but it definitely feels faster.
[robg adds: Make sure Safari is not running when you edit the preferences file, otherwise your changes may be overwritten the next time you quit Safari. I added the lines immediately below the <dict> tag, and it seemed to work fine. Note that in Dave's post, he explains how this may actually slow real page loading times, but that perceived page loading times will decrease.]

