Note: This is only a quick fix and not a permanent solution (i.e. a theme update might reset these changes).
The enlightment theme uses a slider called “flexslider”. To alter the speed you can go into the flexslider javascript-file and change the slideshowSpeed parameter.
In your theme folder, navigate to the file “jquery.flexslider.js”.
It is located at “enlightment > core > js”.
Open the file in a text editor and on line 1073 you’ll find the default settings for the slideshow speed:
slideshowSpeed: 7000, //Integer: Set the speed of the slideshow cycling, in milliseconds
Changing this value should allow you to set your own speed.