Generally a wave is caused where there is a disturbance in a medium and a restorative force overshoots restoring the disturbance
( Electromagnetic waves are a bit different so we'll ignore those here )
If you pluck a string on a guitar you are causing a disturbance and it is the tension of string whic is the restorative force - the more tension the stronger the force - the more force the faster the string will return to (and overshoot it's original position) so the frequency of the wave will be higher - That is the number of times a second it goes back and forth will be higher.
A bit confused - as I think are others here because normally when we talk about the speed of the wave we are talking about the speed of motion of the wave form itself which is zero for a standing wave.
I gather you mean the speed of the string as it goes back and forward. We normally refer to this by it's frequency because although it continues to go back and forward at the same number of times a second the distance it moves slowly becomes less and less as there is a "damping" effect as the string loses energy.
You can tell this because as the string is plucked the note remains the same - it doesn't get lower or higher - this is the frequency.
However it slowly gets quieter - the amplitude reduces
I don't think you're looking for a treatment in this much depth but in case I'm wrong check out this:
http://www.owlnet.ric...Files231/monocord.pdf