How to slow down this Marquee II script ?

1) Script Title: Cross Browser marquee II

2) Script URL (on DD): http://www.dynamicdrive.com/dynamicindex2/cmarquee2.htm

3) Describe problem:
Hello Everybody :)
I have a "lttile" issue with this script. On some computer, people tells me the script is too quick even with "1" speed value.

How could i slow down more this script ? Example : i would like to slow down the script by an half of the speed 1.

As i am a newbie for javascript, i have tried some value like "0.5" but it doesn't work. How to slow this script to have slowest speed than "1" ?
Thanks for your help :)

Xavier
PS : sorry if my english is not perfect, i'm not english native :)