I think the inability to do this seemingly simple customization (changing transition time length) is a notable shortcoming of Zenfolio. Yes, I could build an external home page with my own slide show video, but that is very inconvenient compared to the wonderfully easy and mostly well-integrated Zenfolio system.
Since the Home page is the first thing visitors see, and the slideshow is the obviously dominant visual element, it seems very important to me that it be as beautiful as possible. Transition time duration makes a big difference in "feeling" or "mood" of the slideshow.
Do others find this an important deficiency of Zenfolio?