This seems to be a common problem with possibly many different contributing factors which all need to be checked.
Mine is pretty light now and I think the main problem was at one stage I didn't have the throttle grip gear lined up to the right mark on the little gear in what you call the twist grip gear box. Check this first!
Are the little gears worn? Mine are and it makes the throttle 'notchy' to turn.
Clean and lightly lubricate the handlebar and twistgrip.
Check cable routing, does it get heavier when you turn the handlebars from lock to lock.
Are all the cables and splitter boxes in good condition and lubed? A frayed cable will make things tight.
Does the throttle shaft through the butterfly turn easily, remove the springs and cables and check.
You may need to service the carbs and put new 'o' rings on the throttle shaft but be absolutely sure you seat the butterflys correctly on the shaft or you will never get it to idle slow again.
Does the rubber twist grip bind on the switch block? I finally fixed this recurring problem by making a large teflon washer that fits between the twist grip and switch block. I also purchased a new set of 'E Z turn' or something like that throttle return springs which didn't help a lot initially untill I sorted all the other problems.
Search this site for a previous 'heavy throttle' thread with suggestions and the link to the throttle spring manufacturer.
Cheers