A stiff twist grip can be most tiring to the rider, and will give the impression that the engine is sluggish. It is advisable therefore to get it to work as freely as possible without actually closing itself when released.
An adjustable spring friction pad is incorporated in the twist grip and should the grip work too stiffly (or too loosely) adjustment of the small screw which is just below the cable stop will usually cure it.
If the grip is still stiff after slacking off the friction pad suspect that the inside of the grip is binding on the handlebar, and remove the grip for lubrication.