This extension calculates the time for reading an article. It uses calls to www.diffbot.com to only take in account the real content, it doesn't count words of adds , comments, etc. In the options page you can change the speed reading. You can activate too the "Automatically Load" mode. This mode reads in background every page you visit and if the time for reading is greater than three minutes it shows you this time. In this mode the extensions do a call to diffbot.com for every page you visit.