Hi
I don't use MixPad but I am pretty sure why:
Playback speed is the speed it plays for you, it does not change the speed of the actual clip you are trying to change.
In other words, if you are watching a dvd say, you have the option to press "fast forward". This will make the movie play faster but the file's speed has not changed. That is what "Playback' is, how to play the file. I think you need to right click or something and look for an option that says something like "Clip speed". If it is a percentage, then 50% will be half as fast (twice as long to play) and 200% will be twice as fast (half the amount of time to play). You want it to be faster, 2 out of 3 minutes is 66% so you want it to play 33% faster so you put in 133%.