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Hello I will help you with your question,
It sounds like the throttle plate is closing slowly. This is a motor driven plate so it may be dirty and sticking so it does not close properly, if it is you can clean it. If you are uncomfortable have the shop clean it for you.
Other than that check for any binding of the throttle pedal, if it returns normally the throttle plate should as well.
Let me know how I can help you with your questions