Like the term God fearing, the meaning of God's speed is not so easy to grasp. After all, what speed is God travelling? And where is God going?
Usually the word 'speed' means the rapidity of movement. Now God, existing in a dimension beyond our concepts of time and space, really doesn't need to bother about speed.
But we do.
So when we say "God's speed" to somebody, we are not implying any speed that God might be travelling. The term is a contraction of a longer phrase, wishing somebody a safe journey. Something like saying to a voyager "May God speed you on your journey" prior to a potentially perilous trip, wishing that God will aid the traveller quickly to his destination before any disaster has chance to happen.
Conversely, travelling too fast might be just as dangerous. The voyager might hit a storm that would have been avoided by dropping sails for a while.
God, being omniscient, knows the best speed a traveller should take for a journey, and it's that speed we wish our friends to travel.