I assume you are talking about a '2-stage' return to idle,(normal) not a constant high-idle, which would be a fault to be corrected.
The Year Matters.....kind of.
The Idle Speed motor (programable) offers a range of about 20-25 'steps' to achieve idle. When a Gen-II car lifts full-off, the idle drops in two pre-set steps, with a very annoying delay of about 2.5sec between these two steps.
The reason is to improve emissions, and cat life, and to eliminate burbling and backfiring, (that some folks actually whined about!) that occurs if you could lift off fully.
I especially find the 2-step idle a problem on track......when I want FULL LIFT NOW and not again a bit later ! When driving on track at limits, and the car lifts again , it upsets my already mediocre balance and the rear gets light!
[ I wonder if Dan Cragin, HMS, and ***** can program that OUT when re-programming the ECU for fans, skip shift, etc.? That would inspire a few more ECU sales! Lets talk!]
JonB