There is a simple way to speed up iPhoto considerably, especially if you are running it on an old machine.

Open up iPhoto and go to the Preferences. Choose Appearance. If you uncheck Outline and Drop shadow, then slide the background all the way to white, you should notice that iPhoto starts up a lot quicker. This mainly helps with iPhoto 5's performance, as iPhoto 6 is already much snappier.

If you still find iPhoto to be slow, try holding down the command and option keys while its starting up. A diolog box should load up, asking if you want to rebuild your photo library. This has the greatest effect if you have a very large photo libary.

One final tip is to press 0, 1 or 2 while in your library. This sets your photos to one of the 3 default sizes, which load a lot quicker than the custom sizes you get when dragging the slider.

Now your iPhoto library should scroll like butter!