How do I change the transition duration of a fade animation on a gallery slide?

How do I change the transition duration of a fade animation on a gallery slide?
Changing the duration not work with the settings shown in the following images.
(I want to slow down the speed of the fade animation,
not the stop time per image.)

Can I write code in Velo to change this?
I am a complete beginner in javascript.
Please, someone help me!