Well, the way I see it, I'd have to say it's the voltage going to the motor (or acceleration ability as you put it) that is directly proportional by default, unless it's programmed with a curve on the ESC or the controller (most likely way).
As quick RC cars are to reponding to controls, the actual "speed" of the car will never be proportional to the trigger, just because there's inertia of the car's weight resulting in coasting, whether it's moving or stopped, friction (or lack thereof) of the tires and surface, drivetrain, motor, etc. Eventually it will catch up to your trigger position, but won't be instant.
To be overly simplistic, just go down a straight at full speed, and let go of the trigger. The car's speed will coast down to a stop eventually, but the engine's power is indeed at zero, just like your trigger indicates.
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