Too funny ... a common complaint on a conversion project is the RPM "hanging", or taking too long to return to idle ... you are having the opposite experience.

OK ... couple of assumptions ... that you are using a tune for a manual transmission vehicle, and that you have a clutch switch wired to the PCM (so it knows when you are clutch in / out while changing gears).

If so, then go to the "Idle" folder in the XDF. Take a look at the Rolling Idle and Throttle Follower parameters. You may want to add a small amount of airflow, or slow the decay rate of the removal of Throttle Follower air (extra airflow).

If this does not cause the RPMs to settle more slowly to idle (your goal), then take a look at the Throttle Cracker parameters.

Generally, a small change in airflow may make a significant difference. Proceed slowly, and be prepared for the RPM to "hang" if you go too far.