Speed lock is a term that applies to an anti-cheat measure that affects fast-moving Agents. If a measured speed between actions exceeds 60 km/h (or a little over 37 mi/h), that action will fail.
Possible Mechanisms
Niantic will never talk about any anti-cheat countermeasure they have in place. However, Agents have speculated on how this works.
An Agent's position is recorded with actions (ex. deploying Resonators 
or Portal Mods). The steps can be described as followed:
- Record Agent's position with a successful action.
- When the Agent attempts a new action, grab their new location.
- Using both locations and the time between actions, calculate their speed.
- If it is over 60 km/h, deny the action. If successful, move to step one.
Reddit user, jecowa, posted about the bubble effect, also known as the ripple speed limit. This is a visual interpretation and works well to understanding the approximate mechanism of speed lock. When an action is performed in-Scanner, a figurative bubble is formed from the Agents's location that expands outwards at 60 km/h that vanishes after 15 minutes. Traveling outside this bubble will prevent the Agent from performing any actions.