My car, a BMW M240i, will idle at about 1300rpm after every start. The engine then will slowly decrease the revs until they reach ~750rpm. The reason that the engine is programmed to do this-regardless of the engine temperature-is to ensure that oil is getting to the turbo quickly.
Speculation. I agree with jeepman and edyvw, the main reason is to heat
up the cats quicker. Same on most if not all gas cars last two decades.
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