No, killer whales are not the fastest marine mammals. While they are known for their strength and agility, their top speed is estimated to be around 34.5 miles per hour. This is impressive considering their large size, but there are other marine mammals that can swim faster. For example, the common dolphin can reach speeds of up to 60 km/h (37 mph), making it one of the fastest marine animals. Additionally, the Dall's porpoise can swim at speeds of up to 55 km/h (34 mph). So while killer whales are formidable predators, they are not the fastest in the ocean.