I'm not proposing this as a final solution as the panel may not stay around or may use a different secret keyboard shortcut to open in future, but type the number 42 into the viewer and then turn up the Inertia setting.
After that, close the control and click and drag. The speed your mouse is moving is how fast you'll move through the synth.
I think that the question for the Photosynth team is simply how to make something like adjustable speed simple and intuitive to use especially on a touch screen.
I've also found that I usually prefer using the cursor keys to move through a new synth than letting the slideshow play, even if I am holding down the key to pan in a direction as fast as possible.