Hi, a Chinese CDI does not relate to speed in any way other than it will remove any rev restrictions of the engine if the old cdi was causing an rpm restriction. All Chinese cdi's we have seen use the same timing above idle and therefore there is no performance difference between any unrestricted cdi whatever the brand on a Chinese scooter.
You will see a speed increase if the old CDI was restricting the top speed due to it restricting the maximum rpm of the engine, if the rpm restriction was not an issue you will not see any speed increase.

As a general rule a 50cc 4-stroke Chinese scooter is capable of a genuine 38-39mph absolute maximum the engine simply does not make enough power and is not capable of revving high enough to go any faster.
If you are already seeing this speed on a standard 50cc nothing will increase it unless you start to increasing bore size (cc) and gearing ratio.
If your bike does not make this speed it could be restricted by the original CDI having a rev restriction, by a restriction in the variator or carb or simply by the engine being in poor condition/ unserviced.