Cut down on the CSS3
Drop shadows and gradients look great when you use them correctly, but when your document is 600,000px tall, they create serious browser lag no matter how modern your hardware may be. You get the fastest renderer performance with simple solid colors. Aesthetics are a small sacrifice when you’re trying to squeeze more speed out of your markup.
Hide unnecessary things
I keep all of my calculator’s comments on a single page because 1) that’s the way it has always been, and 2) SEO. However, many users never even look at those comments. It improves browser performance if you simply instruct the browser not to display most of the markup until they are requested, which brings me to the next point..
Limit the number of tree children
Rather than applying a universal
.comment-hidden class to all older hidden comments, put hidden elements under a single unifying parent and apply styles to the parent instead. It’s much faster to style one parent element than a thousand children.
The comments on RogerHub no longer support Gravatar-based photos, since literally hundreds of unique avatars were being requested and rendered on each page load. Since I didn’t want to take out the images entirely, everybody now gets a generic anonymous avatar. Much of the original comment markup has also been taken out, leaving only the bare necessities required to properly apply styles.
Use simple, non-nested classes
I don’t have any hard data behind this practice, but intuitively I feel that simple unnested classes are the fastest way to go about styling an arbitrary number of elements. Avoid tag selectors, deeply nested selectors, wildcard selectors, attribute/value selectors, etc.