One of the major reasons why people click the back button, after finding a site via searching the web, is because the site loads to slowly. Google uses performance as one of its algo keys in ranking sites. How many visitors hit the back button and return to Google is also measured but this data is very quote noisy for ranking sites – none the less it is available to Google for quality control of other algos used to rank sites.
Flash is a major cause of sites loading slowly. HTML5 and Ajax provide alternatives to flash. By using Ajax with animation, which provide a media rich web experience, resources are delayed until the basic page is loaded and the browser becomes responsive.