iframes are good for allowing content to loaded and changed without redrawing a complete new screen. They can be used with a tab menu to make some nice UI experiences. Facebook is going the iframe method for applications.
The html of a page loads around an iframe so it appears faster, iframes can also be used to load scripts in a more asyc manner. But the bad – iframes themselves have a negative total effect on page rendering. They block the onpage load from being executed until the iframe is also loaded. The status bar indicates the page is still loading until the iframe is completed – even though the page renders around the iframe – the appearance is the page is slow. When one looks at the status bar it is still loading.