Are WordPress Heartbeat API ‘beats’ staggered or do they occur simultaneously for all users?

The WordPress Heartbeat API uses admin-ajax.php to run AJAX calls. If User X leaves their browser open, many calls will be made to the server, with each call happening when there is a ‘beat’. Now if User Y leaves their browser open, many more calls will be made to the server, with each of these calls happening when there is a ‘beat’. It’s possible a large website will also have User A, User B and User C all do the same.