Lightweight and responsive
The simple thing is that the lighter the shoes, the less weight swings at the end of the pendulum; your feet. Lighter in a race is always better, but you do have to make sure you train to run fast in lightweight shoes. Lighter shoes tend to also give you less guidance and support, meaning a lower level of protection. Speed shoes were made to go fast on race day or for a speedy workout.
Carbon or not to Carbon?
That's the question. Most speed shoes carry a carbon plate in between 2 midsole layers to provide a higher level of energy return, like a spring blade. Almost everyone benefits from carbon plated running shoes. Combined with bouncy foam types they help you fatigue less quickly during races and fast training sessions. Most efficient runners, like athletes, benefit most from these shoes. The slower you pace, the less impact the shoes will have on your speed, but you still may benefit from reduced fatigue in your legs. Carbon plated shoes are targeted against lighter neutral runners who are going fast, no matter what distance.