I want my character to jump faster not higher. it should go up fast and come down fast. But increasing the jump speed it make it cover a lot of distance in y axis too!!! so how can I make it jump faster not higher?
You have to modify the max falling speed and the gravity, if your character jump fast, it needs to fall fast too 
It seems that if you increase the jump speed by a factor X: Increasing the max falling speed by a factor of X and the gravity by a factor of X^2 keeps the same jump height 
nothing much has changed instead it has started running fast instead of walking.
You need to change the max falling speed, not the max speed.