: Usually, advanced meditators try to use what is appropriate for the moment. When there is sleepiness in the mind it is not appropriate to use three-phase walking. The three-phase walking is more appropriate when the mind is in a state where it is more focused. But that doesn't mean that it is better than walking faster if there is sleepiness in the mind. Actually, walking faster is better for the sleepy moments. Just know what is appropriate for the moment.
Some people find that focusing in too much is not useful for their personality. So we also have to see what type of personality we are, and see what is appropriate for the moment. Especially, say, for a person who lives a very busy life, they have to learn how to be mindful in fast activities. So perhaps that person is practicing walking faster so that they can flow it more in their everyday life, being aware in a faster way.
So it really depends on what is appropriate that determines which is better. Certainly if there is a lot of restlessness in the mind it is very helpful to slow down, bring up more mental noting, and try to be more minutely aware of the feet. This helps to bring forth for more concentration. So the three-phase walking at that time may be more helpful as well. Usually, in the new students' retreat I talk about this aspect of knowing what is appropriate for the moment. Bringing up more mental noting when the mind is restless helps to break through the extra thoughts in the mind with more mindfulness.
Also, sometimes if one is angry and is walking heavily and we notice we are walking faster, then slowing down will often put a bit of a break in the mind, especially if we can lighten our steps at that time. It helps to work with that hindrance of anger.