Soccer reaction speed is defined as the ability to react to a previous action as perceived by the player..
We typically think of reaction and its speed in the context of goaltenders making saves. And that is true as goalkeepers are trained to react to the motion of the shooter and the flight of the ball immediately after it leaves the foot. But all other positions rely on reaction speed as well. Although a player has perceived and anticipated the next play, and made a decision on what to do, the actual play may be somewhat different. Therefore one must react to the actual outcome of the anticipated play. A good example is a forward getting sent into the penalty box with a through ball. Both forward and passer perceive the space behind the defense. The passer anticipates the run and the forward anticipates the pass. The decision is to pass and the forward decides to shoot on goal on the first touch. At the moment of the shot a defender slides in from behind to block the shot. The forward now reacts to the new situation, controls the ball to the side past the defender, and then shoots on the second touch.
Many factors impact the reaction speed of a soccer player, arguably the most important one is aerobic fitness. As individuals become tired, the reaction speed slows. Other factors are the type of reaction, age, gender, motivation, emotional state, intensity of the situation, muscles involved, etc. So it is a very complex process and training has to be very specific to develop reaction speed.
Some soccer drills which are included in our books to improve reaction speed are:
Soccer Reaction Drill 1
Two players face each other with a ball in between them. Distance from player to ball is one step. On command by coach, both players try to pull the back with the sole of their foot.
Soccer Reaction Drill 2
Attackers and defenders are in a grid, attackers with a ball. The distance from grid to goal (with goalkeeper) varies depending on the age of the players. Attackers and defenders are numbered. I suggest to have a maximum of four attackers and defenders paired up, so you have attackers 1,2,3,4 and defenders 1,2,3,4. If you have more players set up additional grids. Inside the grid the attackers dribble the ball and the defenders follow them. The coach calls out a number between 1 and 4 and the attacker whose number is called immediately tries to leave the grid and go for a shot on goal. The defender who has been shadowing the attacker must react and try to prevent the shot.
Our books have many more reaction speed drills incorporated into practices. Our goalkeeper book, Soccer Goalkeeper Practices has a huge focus on reaction drills.