Classic and Developer modes do not change backgrounds in "regular interval mode" at all. They just stay at the same background. My backgrounds are 256k each and are uploaded at /wp-content/backgrounds/ . I have no plugins installed except the WordPress Importer, Broken Link Checker, and Regenerate Thumbnails.
This would be fine if "Better Background" mode let me choose for the backgrounds to not resize to fit the browser window. The way my backgrounds are set up, I need them to be the same size at all times. The backgrounds give information on some of my site's projects, and certain parts of the background need to be in the site banner at all times.