Effectively yes. You have to remember that when crossing the planet, Superman ascends to ridiculous altitudes where there's essentially no air. The only reason jet fighters can't accelerate infinitely is because of air resistance—the force of the thrust becomes equal to the force of the air pushing back on it, which is why the velocity of a jet might vary by a whole mach number based on its height. That's terminal velocity, and the forces involved with an object moving at supersonic speeds through atmosphere are rather intense, and the power output required to maintain that velocity is even more intense. They are also not capable of ascending into space.
Air resistance increases with the square of the velocity. So then what kind of speeds could Superman reach at 128,000 feet or higher if he generates enough constant force for up to mach 2 terminal velocity at sea level?
The answer? Really, really fast. But that doesn't make him any faster down here. Which would explain why can cross the planet in a few minutes off-screen, but on-screen, never exceeds Mach 2 or so.