A lot of factors affect the difference in performance between desktop and mobile applications: network organization (LAN/WAN), CPU architecture, etc.. Therefore longer fetch time on Android in comparison to Windows applications is quite natural.
The TMyQuery component has the FetchAll and FetchRows properties, which default values are True and 25, correspondingly. If FetchAll = False, only FetchRows of rows will be read during the fetch of the TMyQuery.Open method. 
The rest of the rows will be fetched by necessity. 
You can read about it in the MyDAC help: 
http://www.devart.com/mydac/docs/devart ... tchall.htm