I know this would likely vary in different incarnations of Superman, but is it ever specified how far Krypton is from Earth? And is it ever placed on a star chart so we have an idea about where it is?
I started thinking about this when someone made a comment about Superman II and I remembered that a test of a nuclear weapon from Earth created shock waves that ruptured the Phantom Zone and let Zod, Non, and Ursa escape. Shock waves would be sub-light and we don't know how fast the opening to the Phantom Zone travelled as it wandered through space. We also don't know how fast the ship Kal El arrived on Earth was capable of travelling.