Bulbapedia and Serebii say it's 25% but I've seen some places mention 50%.
Paralysis drops your speed to 25% of its original value, so it drops it by 75%. Maybe you got the 50% from the Ability Quick Feet boosting speed by 50% when paralyzed.
their speed is reduced to 25% of their normal speed. if the Pokemon would happen to have the quick feet ability then it is increased by 50% instead