The throttle slides will not open instantaneously when you twist the throttle. They open based on vacuum. Now if you have a vacuum leak, a dirty throttle slide or a worn throttle slide that affect the operation of the throttle slide going up and down.
Vacuum leaks can be caused by torn diaphrams, caps not sealing, carbs not sealing to intake boots or a cracked carb housing (this is very rare). Dirty can be varnish build up, dirt or other things. You can check the throttle slide action by lifting with your finger and see how easy it slides up and down.