Lack of power

I have a Generation 8, 1999, Toyota Corolla Liftback 1.6 automatic.
I have noticed for several years that when on a slight incline for about half a mile on a motorway, the car slowly starts to slow down, sometimes to about 45mph though the accelerator remains down. When pressing the accelerator still further the car suddenly goes into a higher gear but with little improvement in soeed. Is this a design thing or because of the age of the car it has simply lost power over time?