I think the fastest way is to travel by plane because plane travel through air and it fly across any obstacles such as sea, mountain, building,etc. So it can save time when we want to travel abroad.
By the way, I think travel by sea is the slowest way but they offer more entertainments and space to walk around. As you can see, the luxury furry that has swimming pool, casino, restaurant, etc. So many people prefer to travel by sea instead by air.