This is a very typical misconception that exists among most observers. Bolt does not slow down for the most part, but appears to be slowing down. So what explains this apparent visual incongruity?

So, let’s try to clarify why Bolt is so phenomenally fast in the first place. The usual answer is that he won the genetic lottery. Not so! In fact Bolt was born with scoliosis, a condition which causes bending of the spine and led to Bolt having his right leg half an inch shorter than his left.

According to a study by Southern Methodist University[1], uneven strides are supposed to hamper speed. Bolt apparently hits the ground with his right leg exerting 13% more force and staying on the ground for 13% less time than his left leg. (This is because his right leg has to fall a longer a distance thereby acquiring a higher velocity.) According to researchers, although Bolt has optimised his running gait for performance, he could possibly have achieved higher speeds with a more symmetric body structure. Where his genetics does come in handy is his height, which at 6 feet 5 inches leads to Bolt taking 2 to 3 steps less on average in completing the 100 metres.

This still does not explain this:

According to a Harvard study in 2000, the research group concluded that human runners reach faster top speeds not by re-positioning their limbs more rapidly in the air, but by applying greater support forces to the ground.[2] Thus, from a mechanical engineering perspective, it would be more efficient for Bolt to exert equal forces with his legs to jump forward, rather than the unequal forces he is forced to apply because of his genetic scoliosis. So, that clearly means, a faster Bolt protege is probably possible in the near future.

Further, most sprinters don’t run at top speed for the whole length of 100 metres. There are 3 stages: acceleration, constant velocity and deceleration. This is what a the chart looks like when Bolt is compared with other sprinters: