Rapid is a Smithing and Firemakingperk which has a 5% chance per rank to speed up an action by a game tick (0.6 seconds). This perk stacks additively with Varrock armour 2 or higher when smithing, but they both can't activate at the same time to stack speed times.[1] It can be created in tool gizmos.
When the perk triggers, it prompts the message: Your rapid perk speeds up the action process.
Effective time saved and action increase
(Level 20 in parenthesis)
^Mod Ryan. Rapid Perk + Varrock Armour for smithing. 6 February 2017. "The speed doesn't stack, but the chance of it happening does. E.g. If you have both the armour and perk, it's more likely to proc."*