Using the offline data entry mode is recommended for performance reasons. 

When the offline data entry mode is activated, the journal of pending changes can be viewed. Right-click on a write-enabled cell, select Anaplan XL > Properties > Writeback tab and the Manage… button in the grid's properties window.

All changes can then be saved or discarded at once by using the buttons in the Editing ribbon.

You still need to be connected to the underlying cube/model to use the offline data entry mode.