Okay first thing is Superman can fight at super speed. He's used it, albeit sparingly. Like when he fought Mongul. But the thing is he has shown he can move and react at super speed, it's silly to try to argue he can't throw a punch at super speed. It makes NO SENSE whatsoever. If all he did was fly fast you'd have a point, but he doesn't.
But it doesn't mean Doomsday can defeat Sentry, who is far above him at his strongest.