From Fedora Project Wiki
Re: "nearly ok, but too long black before fading in" for Fade/Cut Through Black transitions: Both of the transitions show the outgoing slide for 1/3 of the duration of the transition, then black for another 1/3 and then the incoming slide. In the preview mode there is no outgoing slide, so black is shown instead of it, which means that black is actually shown for 2/3 of the duration. When tried on transitioning from first to second slide in slideshow mode, it works fine.