You don't need to modify any file like slideshow.js . You can set Slideshow Transition Timing from TemplateToaster UI.
1) To set the transition duration for background images, go to TemplateToaster -> Slideshow -> Transition effect -> Transition duration (in the bottom) -> More -> set the custom duration
2) To set the begin and duration time for foreground images, go to Slideshow -> Foreground images -> Transition effect -> Transition duration and begin time.