How to change the transition speed between slides?

To change the transition speed, first hover over the gallery widget and click  the pencil icon. Then drag the interval bar to the left or the right to choose the speed. 

Here is how to change the transition speed between slides:

Step 1: First hover over the gallery widget and click the pencil icon. (How do I enter a widget's edit mode?) 
Step 2: Drag the interval bar to the left or right to select the transition speed (a).
Step 3: Click Done (b).

Below is an example of an interval of 2,000 ms.