Every single engine is different
It depends on its maximum torque peak in the rpm curve, that is where it is most efficent
meaning on a constant amount of fuel and air with spark it gives out the best results at that rpm, horsepower is irrelivent as it is only the engines ability to spin faster not move anything.
So bsing below 2000 rpm might be good for some cars but not many as most don;t make there max power way down low like 200 rpms