Even with vba the only speeds available are slow, medium and fast.

You can however make a slower cross fade by saving the presentation as jpegs
and then inserting slide1.jpg over slide 2 for example and giving it a fade
exit of any speed you like or duplicating the first slide and having the
content of slide 2 fade in over it

Hope that makes sense!
Sounds more hassle than it is and can work very well.