Okay, there are two field that you need to keep in check.
Stats - Casting Time
This refer to the damage done per X (X = Casting Time).
If you set it to 3.00, it will damage the unit per 3 second each.
Data - Decay Power
This is the speed reduction (AS and MS) per X (X = Decay Power).
If you set it to 3.00, the units will get a maximum of slow (AS and MS) at first cast, and then for each X second(s), the unit's speed will update over time based on the Decay Power.
Example: You have been slowed by Shadow Strike for -50% AS and -25% MS. The data for Decay Power is 5.00. The spell lasts for 15 seconds. Each 5 second, the unit's speed will increase by half per X second (Decay Power)
Initial Slow - AS = 50% | MS = 25%
Second Slow - AS = 25% | MS = 12.5%
Third Slow - AS = 12.5% | MS = 6.25%
Since the Decay Power is 5.00 and the spell duration is 15.00, therefore (15 / 5 = 3), there will be 3 interval in the spell per cast to update the unit's speed.
NOTE: Same principle applies to the default Blizzard ability, Purge.