Currently, there is no way to change the default for the slide show duration. Nor is there a way to change the duration for multiple clips/images at once.
We do have a feature request to add this capability, though.
In collecting requests for this feature, it's always helpful to have customer feedback about what your workflow is ("I'm making 15-image slideshows.") and how much the feature would help ("It would save me 30 min of work and 45 clicks if you implemented this feature.) If you have a moment, would you please add your 2 cents as to the importance of changing the default/being able to change them all at once?