People often argue that if Superman was fast enough to spin the Earth backwards then why was he not fast enough to catch the rocket?
And with Superman Returns...the plane rescue. I think it's all for a matter of timing. Yes Superman could have stopped that plane from falling easily, or the rocket, but I think if it happened like that, it would be boring and not fun, therefore they make it longer, for more story and excitement. I don't have a huge problem with it.
The way I look at it, Superman can always get stronger, it's only a matter how he will use his strength to what length. Superman constantly has to be careful with his strength. He can hurt someone easily. In Justice League: Unlimited, Superman showed his true power. He was in fact, capable of beating Darkseid all along, he just didn't want to go that far before.