ZLC is softer than ALC, but is less flexible. The ball sinks in more easily to the softer surface, but the ZLC layer does not flex and the ball bounces out more directly (faster).See the graph, BTY ZLC are always fastest than ALC.
ALC is harder but is more flexible, the ball does not sink in as much but the ALC layer will also flex and give more of an arc but not as fast as ZLC.