There are no controls to slow down the slideshow speed in a photosynth or the panning speed when using the 'Play' button on a panorama.
Here's a similar request to yours from 2008:
In photosynths, you can use the spacebar or the period/full stop key to proceed to the next photo at your own pace.
In panoramas, you can switch panning modes (second icon from the left at the bottom of the viewer), click and hold with your mouse, and then move the virtual camera around the panorama with your mouse. The nice thing about this secondary mouse mode is that you can control how fast the panning is by how far your mouse cursor is from where you originally clicked.
Hopefully that helps.