Iframes, also called inline frames, allow for one HTML document to be embedded inside another. Iframes are best used for integrating HTML content, such as an ad, that’s from a web site different from that serving the main page.
What’s the downside? You guessed it—slower performance. Chapter 4 describes how iframes are used to improve performance in terms of loading scripts asynchronously. It is true that iframes can make pages load faster, if used properly. Unfortunately, iframes are often used in a way that hurts performance. It’s important to know the performance penalties inflicted by iframes and how to avoid them.
The Cost of Elements test page measures how long it takes to create different types of DOM elements.
Using this ...