Catching someone who is under the speed power-up can be difficult when you are on foot. If you are an engineer you can use either Hyperspeed Accelerators or Nitro Boosts to get to them. Rogues can sprint, Hunters can use Aspect of the Cheetah to move faster. If you happen to be close enough when they first pick it up then Warriors can Charge or Intercept, Paladins can use Hammer of Justice, Death Knights can Death Grip and a Mage's Blink might get you close enough to frost nova them. The speed power-ups can be found in the Horde and Alliance tunnels close to their respected Flag Rooms.