Fin blades are made of a variety of materials. Carbon fiber is undoubtedly the lightest, most flexible and most responsive material.
However, efficiency is affected by the stiffness of our carbon fiber fins. The stiffness of the blades should match our musculature; too soft fins will give us less power and require extra movement, and too hard will take more of our energy, burn more of our precious oxygen, and can also cause more lactic acid buildup.
Nowadays, freedivers and spearos use softer blades which fatigue less while swimming and in turn maintain good technique and prevent the knees from bending.
Soft fins do not require much strength from the diver and hence are generally more efficient for thrusting into the water with less effort. A good technique is the key and if you achieve this, you can use soft blades.
Speed is another important consideration. You swim faster with stiffer blades but you may tire more quickly. Hard fins require that the diver has strong legs, in which case they can provide a great amount of thrust (e.g. the kind of thrust needed during ascent while fighting a fish at the tip of the shaft). Harder fins might be useful for spearos who are diving in strong currents or for experienced swimming athletes with very developed muscles and strength in their lower bodies.
Using stiff, long fins is challenging if you are just starting to freedive or spearfish because a lot of energy is expended trying to keep your legs straight and learning the correct technique. They are therefore not recommended at this point.