Colonists travel across open sea much faster than through land or coastal sea provinces. For example, European colonists can reach the Caribbean faster than Sub-Saharan West Africa, even though it is physically much farther away.
Colonist travel time is a fixed value based on the rules explained above; no events or ideas modify it. Colonists return instantly the day after either the colony becomes a city or they are recalled.
As soon as any nation chooses to send a colonist to a province, no other nation can send a colonist to that province unless the colony is later destroyed before completion, even if the colonist will actually arrive two years later. Two nations cannot 'race' and see which colonist will arrive first to the province.