You have the basic idea right. Custom speed allows you to keyframe how fast or slow the animation goes. However, you need to also pay attention to the spread parameter. Spread will expand the animation past the word that's currently being animated. For example, with the Scale In behavior, the 2nd word starts it's animation before the 1st word has finished because the default spread for that behavior is 2. If you want to ensure that each word has it's own animation speed, you need to set the spread to 1.
From there the custom speed slider is pretty easy to understand. It goes from 0 to 100, an indication of the percentage of animation in the effect. Place keyframes at places in the timeline where you want each word to animate on. Then adjust the slider until that word is animated. Wash, rinse, repeat.
Andy
http://www.timesavertutorials.com