Over the course of the past few months, Dashboard had become painfully slow for me, sometimes taking 10 to 20 seconds to load.
I took a look at ~/Library -> Caches -> DashboardClient, and found that the Dashboard cache was 20MB. This seemed excessive (plus, I had a fairly recent backup), so I took a gamble and deleted all the files in the DashboardClient folder. Dashboard now loads almost instantaneously for me.
[robg adds: I was doubtful about this tip, but it did make a difference in my Dashboard loading time -- even the first-load time after a reboot was notably reduced.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061002101155933