There's no way to change the default speed of the panning of the panorama when using the Play button [>].
You can use your mouse cursor (or fingertips on a touch screen) to manually drag the panorama at a speed that you prefer.
If you are using the Silverlight viewer on a laptop or desktop, there is also a second mode of control for your mouse that you can switch to via the left side of the lower bar of icons. When you switch to it, you can click on part of the panorama and as long as you don't lift your mouse button up, the panorama will continue to pan. The speed at which it pans will be determined by the distance your cursor moves from where you first clicked with shorter distances yielding slower speeds and larger distances yielding faster speeds.
Lastly, you can also use the cursor arrow keys on your keyboard after you've clicked on a panorama to pan around.