It's 4am in the morning so try to give a little slack on what follows... And please, correct me if I am wrong with any of this.
Firstly, check to see if it is a "cast time" vs a "cast speed" mod. There is a big diff between the two.
Casting time is the simple one, a 20% cast time mod would make a 10s spell, take 8s to cast.
Casting speed is the pain in the rear. Casting speed mod was best explained (that I seen) using a car going from point A to point B as the example.
Assume your casting bar (point A to point B) is scaled to represent 100 miles. And also assume your default speed is 100 miles per hour. It will take you one hour to get from A to B (point of casting to the point of finishing the spell).
If you increase your speed by 50%, you are now travelling at 150 miles per hour. This will now take you 40 minutes to travel 100 miles (point A to point B). So for a 50% speed increase, your spell cast time (travel time) drops to 66% of the original value.
If you have a mix of cast time and cast speed mods, it can make it look a bit strange. Hope this helps.