ArcGIS Online by default loads sub-layers (ending with a number like /19) as feature layers which are slow and limited in number of features. If it supported adding sublayers as dynamic layers, that would help for many reasons, including when ability to re-order and Group layers in a webmap is added at last. But also, the button ‘Create Webmap’ on Open Data would be useful. Currently it is not useful unfortunately, as most layers have more than one thousand features. (yes you can raise the cap but that hammers the server, and also the browser to some extent with large feature layers) Also once this is rectified I would recommend Add to Webmap on Open Data be more prominently displayed and labeled in plain language such as 'View on Map' etc.