Any rocket can achieve a very high speed if it accelerates for a long time.
A conventional rocket has a hard time doing this because a huge amount of fuel must be carried into space in order for this to happen. This may make the rocket too heavy to lift off. Conventional rockets are generally designed to meet the speeds necessary for them to go where they need to go, and not go much faster. This is the minimum speed for a spacegoing rocket. The farther from the Earth, the faster it needs to go.