Well all we're modifying is how long till out city grows, not its maximum size or anything else. Let's set it obscenely high. Let's say 0 food required: our city is as big as (food output) - 2*(city population) the moment it's built. What will happen? What will break completely?
That would be pretty funny. It would break the happiness system I think. Founding such a large city would almost certainly send you below -10 unhappiness, so you'd need to save up enough gold to immediately rush buy happiness buildings to counter that. Otherwise you're stuck building coloseums at -50% production.
The early game would be hurt the most. You wouldn't want to found any extra cities, so you'd just have to sit there banking up gold for many turns before you can finally get a second city.