Working with Excel in office environment extensively needs you to work with keyboard. You need to be quick to be productive. Time and motion study proves it that making use of keyboard shortcuts reduces time required to accomplish a task. You must learn the short cuts for the tasks you use more frequently.
Probability is quite higher that you use autosum option quite frequently. When applied, this deploys SUM function on selected cell such that cells above that are added. If there is no value above, preceding cells in the row are checked and sum of the values is returned.
This task can be done quickly using keyboard shortcut. You only need to hold ALT and press + button. And SUM function as described above is implemented on selected cell. Like Cell A6 was selected and ALT+ is pressed to get sum of cells above selected cells.