Full speed for two hours? I'm no biologist or physicist, but an animal the size of an orca going full speed for two hours just to get one dolphin hardly sounds efficient. The orca may have burned more calories during the chase than the calories it consumed when it ate the dolphin.
I agree with the comment in the article that it appears they were playing with it for at least an hour. It seems they fatigued it enough that they could take turns chasing it so maybe they had little breaks?
To me, the description of the hunt makes the killer whales sound like they are giant swimming ......cats?