The slideshow view is still in an early stage of development with only a basic set of features.

If you don’t need the auto-advance mode, you could even use the sticky preview feature instead.

spacebar             start and stop auto-advance mode which automatically switches
                     to the next images every five seconds by default.

ESC                  leave slideshow mode and return to lighttable view.

+ or                 increase delay between each image.
up arrow 

- or                 decrease delay between each image.
down arrow

left-click or        
right arrow or       switch to the next image of the collection.
right shift-key

right-click or       
left arrow or        switch to the previous image of the collection.
left shift-key

Hint: To take full advantage of your screen size, put darktable into fullscreen mode by pressing F11 and hide the border-controls by pressing the B key.