It's easiest to learn by example. Check out some of these detailed walkthroughs.
How Changing Ad Networks Made My Site 8x Faster
This is a story about how I made a simple (stupid) choice to switch ad networks to increase ad revenue by a few percent. Yet it ignorantly turned a <2 second load time into an average of 15 seconds.
Optimizing Images for Web: A Practical Guide
Richard Lazazzera walks you through optimizing images on your site, average improvement of 30%
Reach out to me (see contact info in footer) to let me know of other examples
Feedback? Contact me: shane at httpfast dot com