Endless Slideshow Screensaver offers more than 200 transition effects for your slideshow and you can control how long those transitions are running for. In the screensaver settings select the [ Slideshow ] tab, [ Transition settings ] section:
Here you can use the [ From ] and [ to ] fields to set minimum and maximum time in seconds for every transition to run. If you want all transitions to run for the same period of time - use the same values in both fields. If you set different values - Endless Slideshow Screensaver will use a random number of seconds from the given interval for every transition.