In general every individual has a natural walking cadence (steps per minute) and they vary their actual speed by increasing or decreasing stride length. Even amongst a very fit group the most comfortable natural pace will vary and some people will be built for speed on the flat while some stroll up brutal hills others will happily carry their own bodyweight in kit at a steady pace over any terrain.
However the degree to which you can vary pace is limited and there is a point where your strides are uncomfortably long or (as in this case) short to maintain an efficient gait.
The main reason for this is that natural walking is effectively a whole series of fallings forward ie you aren't pulling yourself forward with your feet you are swinging your centre of mass forward and moving your feet to maintain balance. So your natural gait is a lot like a pendulum and like a pendulum it has a natural frequency.
As this happens you are transferring energy around a bit like a slinky falling down some steps, although some energy is lost each step a natural walking pace will conserve quite a lot from one to the next.
If you have to move too slowly the your gait becomes a sequence of individual steps rather than a smooth flow from one stance to another and rather than a recycling the energy of each step you end up using energy to hold yourself back and check your stride.
This is exacerbated if you are carrying a pack as you end up accelerating and decelerating it with every step rather than keeping your centre of mass moving at a fairly constant velocity.
In practice if you are walking with someone who is struggling it is best for everybody for you to take some of their gear and/or for you to continue at your own pace for a while and then stop and wait for them.
Also if someone is having trouble have a group rest and find out if there is any underlying reason why they are struggling, if they are carrying too much weight, are dehydrated, hungry ete etc it is better for the group as a whole to solve that issue than let them struggle even if you have to push them to admit it. Most people will need to have this explained to them and will not admit a problem unless pressed.