It's definitely a real thing. It's usually referred to as "brake fade" in car enthusiast communities. It’s usually more of an issue when racing, but it can happen if you have to do many hard stops during regular driving as well. I think it has to do with the pads releasing a gas when they get hot which creates a thin layer of “air” between the pad and the rotor.

It's the reason that sports cars generally come with bigger brakes. Sure, a Toyota Corolla might be able to stop in a similar distance, but things change when you have to do it 5 times in a row.

For most people, it’s not really an issue though.