Users expect websites to download quickly and if it doesn't they get frustrated and leave the sit. The amount of time a typical user is willing to wait before quiting the site is known as the threshold of frustration. Research sets the threshold at 10 seconds; that is' if the page doesn't download in 10 seconds, visitors are likely to leave.
Web designers should pay special attention to anything that increases download time. The main culprits are large images. Image files increase download time. Hence we should limit the number of images and size of images we use on a page.