In all games it appears in, Serious Speed is a pair of red sneaks with large red, yellow and black wings on it. When picked up, it greatly increases the player's speed for 20 seconds in The Second Encounter and 25 seconds in Serious Sam 2.
In Serious Sam 1 and HD, Serious Speed first appears in Sierra de Chiapas, after the player first obtains the RAPTOR Sniper Rifle and when the large wooden gate opens. In further levels, it often appears in secrets.
Serious Speed is very useful, as it lets the player circle strafe enemies while attacking them. This lets the player easily avoid attacks and constantly harm enemies. However, when you hear a beep, you should make sure that you're in a safe location before its power runs out. Otherwise, you might end up in a bad position when it runs out.