More information please. Is this new problem? Do you ride all the time? Does the bike sit a lot?. Off the top of my head, I would first lubricant my throttle cables.
on mine I was having almost the same problem, after releasing throttle it would take a long time to go to set idle RPM, just like the throttle cables were sticking . I found that my throttle positioner was going bad. You can verify this by looking at the arm that connects the two banks of throttle bodies together.
Check ECM for trouble codes. A sticking IAC (idle air control) valve would be high on the list along with Tw sensor for further checking after first checking for throttle linkage/plate issues.
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