In both games it appears in, Serious Speed is a pair of red shoes with large red, yellow and black wings on it. When picked up, it doubles the player's speed for 20 seconds in The Second Encounter and 25 seconds in Serious Sam 2.
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 the player hears a beep, they should make sure that they're in a safe location before its power runs out. Otherwise, you might end up in a bad position when it runs out.