I noticed on my YZF that if one of your throttle cables is adjusted too tight, it makes the throttle return very sluggish. The adjustment is like the other guys have stated, under the cover on the left side of the carb. Don't know why this is, but after adjusting the 2 cables back and forth, my throttle returns with a nice crisp snap. Your cables shouldn't be adjusted to where the cables are taught, try a little slack and that will help. Also if your throttle is mounted too far inward on the handlebars, the end of the throttle housing will rub on the end of the handlebar and slow down the throttle return. Good luck,
Pat