Thanks all of you for helping me in getting out of this problem.
Diablo suggestion worked for me. The problem was actually lying in CharacterMotor, it is restricting or controlling the speed of the controller in sideways but the script is so lengthy that I am unable to understand what the code is actually doing but somehow I atleast got to know that the problem lies with CharacterMotor script.
Now I do not want to write my own code for the controller from scratch so can anybody suggest me where can I get a good code for character controller movement?
And thanks again