Minimal Download strategy:
Minimal Download strategy is enhancement introduced in share point 2013. It improves the rendering performance of a page, when there is large parts of the page don't change. So that it can provide more fluid navigation experience to the user.
As general AJAX case, controls uniquely interface with sever. In SharePoint Chrome controls are with the URL at their core. Minimal Download strategy implements a new download manager that interfaces between the client and server and retrieves data as needed depending on the initial request. each and every controls interfaces with DM to update itself.
In share point page we have mainly two parts chrome and content area.
Chrome defined by master page includes overall layout, core styling, page behavior, location and size of the content area and any common controls like navigation which shares across pages
Content Area defined by content page inherits styles from master page and interacts with the controls on the chrome.
Unlike SharePoint previous versions page will not be loaded every time when user request the page. Here MDS will reduces the amount of data need to be download like css, Scripts. So that the browser parsing and rendering time will be decreased ,performance will be improved and proves smoother transitions.
Minimal download strategy uses start.aspx file pages, with URL will be encoded in '#' mark.When navigating two pages only changes will be downloaded. Download manager is responsible for difference between source and destination pages.
Enable or disable:
In Site features we can activate or deactivate minimal download strategy feature as shown in the image. (Site Action->Site features). By default it will be activated.