Speed lock applies when the Agent reaches speed of 60km/h (37 mph) or greater (for example by driving a car). It affects XMP and Ultra Strike shooting. It also denies remote charging. Further detailed information is unknown, however there has been some speculation on the way speed lock works.
Reddit user jecowa posted about the bubble effect, also known as the ripple speed limit. When an action is performed in-game (ex. deploying Resonators or Mods), a figurative bubble is formed from the player's location that expands outwards at 60 km/h that vanishes after 15 minutes. Traveling outside this bubble will prevent the player from performing any actions.
Reddit user Bownage pointed out that Ingress will not track your location if the app is not open. This can allow players to travel quickly within 15 minutes without worry about speed lock activating.