I got the idea for this post from Gwen's blog After the dress...
I have a Viking 500 from about the mid-'90s, and it has speed control. It appears as a stack of lines just to the right of the word SPEED on the left side of the screen.
Set at 2/3 height, this limits my maximum sewing speed to a medium speed. Set at 1/3 height, it limits my maximum sewing to a slow speed. Set at full height, it allows me to go as fast as the machine's maximum speed, whatever that is. At any speed setting, I can still control the speed with the foot pedal, but cannot go any faster than the maximum speed setting.