Ship speed matters in the following things:
-Travel times
-Which ship shoots first in battle
-Battle Efficiency (A fast ship can outmaneuver a slower one - effects of ship speed in battle are right now -perhaps- weak. However, if it is combined with the right upgrades such as Cotton Sails, it can play a bigger role.)
In future we will add specific traits to admirals of each fleet. Ship speed role will be upgaded in these traits. For example you will be able to set an order to your admiral to try and escape battles. The faster the ship, the faster the chances to run away. For example, a cutter due to size + speed will almost always escape
These traits are part of our advanced plan to make sure that players will have a lot of options in fleet building