It works if the service runs a sufficient number of application instances behind a load balancer. In one of the videos about Tornado, a person from FriendFeed stated that FriendFeed runs multiple instances of the application with blocking calls to MySQL. There were many complaints about FriendFeed, but being slow was not one of them. (See slide 21 here http://bret.appspot.com/entry/tornado-tech-talk).