10 to 20%? Some people will spout a load of stuff about how it will affect climate and that will affect actions etc, but the crux of the matter is: not really much at all. There are very, very few events in history where the outcome depended on how long it took to cross the Atlantic. The discovery and colonisation of the Americas are not examples of this, and neither are virtually all of the important events in American history.
Now, if the question was 50%, or far better, 100% or more...now that's a different story.