I assume Missile Speed means X distance/pixels/units per second. You could test it out to be sure. Try starting a 1 second timer, then cast your missile spell (Shockwave for example) 1000 distance away from the caster, and then see where the missile is at the end of the timer. The timer and the missile should expire at the same time.
And Missile Arc is the parabola effect. Arc doesn't interfere with Missile Speed, it just makes the Missile... arc. Test a missile with and without Arc, you'll see right away what it does. To get a better look scroll the camera in so it's easier to see the height of the missile change.
