The average that human can swim is about 2 miles and hour. However when one wears fins, their speed will naturally increase to about 30 to 40%. Fins help remove resistance in water and helps the body use less energy while swimming.
So this makes swimmers increase their speed while in the water. This also helps swimmers to get more exercise in a rather much shorter time frame. Plus using flippers is good for speed training as it helps particular muscles in the body.
So Shorter fins will DEFINITELY make you go faster. The longer the fin, the more water you push.
From personal experience I would say I can swim about 30% faster with flippers on. Why? They add surface area where its needed most, your feet. Humans tend to have strong legs by virtue of walking upright, however this doesn’t translate all that well to swimming because our feet aren’t designed for use in the water and are only mediocre at providing propulsion while swimming. Fins solve this problem, they allow us to bring our powerful leg muscles to bear on the task.