Leo's nervous system is adapted to work with less voltage, starting pulses faster than an average human, making him process information much faster than an average human and being the base for reaching much higher speeds.
His cells are packed with many more mitochondria to accelerate the energy production, and are more efficient, losing less energy as heat and preserving much more as chemical energy. A consequence of this is that his body is affected that much faster by poisons in general, since his body incorporates them that much faster into the metabolic cycles. This means that Leo's condition turns severe very fast, although once he receives an antidote he recovers equally faster.
His tissues are geared towards resisting the wear produced by the fast movement, this doesn't makes him harder or more resistant to other sources of damage other than friction though, thus impacting at his top speed against something could potentially kill him, or end with many broken bones.
Leo is extremely susceptible to electricity due to the way his body works, requiring about 1/5th of the current an average human does to be stunned, and about 2/3rds to be killed, making even a plug lethal to him.
These changes in his body allow Leo to reach speeds close to 50 times faster than an average human being, particularly reflexes and trained actions, letting him reach the speed of sound.
Leo's top speed is accessible to him almost instantaneously, taking him about 0.2 seconds to reach it. His processing speed is always at the top speed, hence why it'd take him such a short time to reach it. This unfortunately doesn't help him to learn or memorize faster, only to process and react to stimuli.
Because of his way faster reaction speed coupled with reflexes, Leo could out of instinct dodge most subsonic and a few supersonic moving objects without major trouble. He is still subject to bluffs and feints though.
If Leo was to duel Wild West style, he'd process the draw command much faster, and thus start the process almost as soon as the draw command is given, then draw, aim and shoot much sooner, giving him the first shot against an average human, and have time to perform several follow up shots before the other could finish its draw.
Using his top speed Leo is capable of hitting faster however doing so also risks his health, given his body isn't designed to bear the blunt impacts. He can however throw many faster hits with average strength rather than strengthen them through his speed.
Leo is capable of boosting his speed to the top as often as he wants and as long as he's not tired yet, doing things at his top speed tires him as much as an average human would be doing things at its own top speed. He is capable of enduring the consequences of most of his trained actions at top speed, such as shooting faster, or running faster, as if he was moving at an average human top speed.
Untrained actions can be performed at half the full speed Leo can achieve, however due to their improvised nature and the lack of experience, a mistake can damage him, proportional to the speed he's using.
Substances with higher density than air force Leo to slow down, in an approximate proportion to the difference of densities, with water returning him to average human speed, as well as the time he could perform any activity down to a half while partially submerged.