Hi Peter,
I looked into this hoping I could add a simple setting for this, but while it is possible to control this setting, it requires taking over the entire configuration for the Vertigo Silverlight Slideshow. In the new 2.3.4.3 release of mojoPortal I made this possible.
See this document for information:
http://www.mojoportal.com/customizing-the-vertigo-silverlight-slideshow.aspx
The setting of interest in this case is "WaitTime"
Note also that I'm fairly sure the slide show pauses if you mouse over an image, this may also help your users.
Hope it helps,
Joe