You need to consider the broader context. In this situation you were polite and gently offered help. I think that’s the right thing to do. You could have been more forceful, which might (or might not) have sped things along but at the cost of making the world a less polite place. You could have ripped the phone out of their hands, which could have really sped things along while being even more rude.
In this case the degree of “suffering” you’re talking about is a few people waiting in line for 2 minutes, so going “Karen” on the person is likely to do more harm than good.
More generally, utilitarian calculations often neglect broader context. One utilitarian might argue that murdering an innocent person for their organs to save 5 organ recipients is good for the world. A better utilitarian would point out that in a world where we could each be murdered for our organs at any time, the increased fear and anxiety probably makes that world worse.