Each transition comes with a default speed that you can adjust. Watch this video to find out more.
Set the speed of a transition
A shorter duration means a slide advances faster, and larger numbers make the effect go more slowly. Select the slide that has the transition that you want to change.
On the TRANSITIONS tab, in the Timing group, in the Duration box, type the number of seconds that you want it to run.
If you want all the slide show’s transition effects to use the same speed, click Apply To All.