The stock throttle is deliberately non-linear for sensitivity at small throttle openings. This is achieved by a cam and chain shaped to vary the response. Maybe you could modify the cam but I've never heard of anyone doing it. Shouldn't be sticky on return though so there must be a lubrication issue somewhere.
Agree - in my experience the two most likely candidates are"-
1./ old grease in the throttle "cam box" or between handle bar and throttle cylinder.
2/. Over time the grip elongates and then jams against the switch housing, usually in spite of the intermediate spacer BMW thoughtfully provided.
To cure the first, strip clean and re-lubricate, the second can be staved off for a while by carefully pulling the grip away from the spacer and the use of dry silicone lubricant on both sides of the spacer, but the real cure is new grips.