How should I handle an employee who eats lunch late and delays the shift change?

Our department is split into Team A and Team B, and we take lunch shifts. Previously, Team A would go at 12:00, and just before 12:25, they would swap so Team B could go. This was our established lunch pattern.
However, a new female employee joined Team A a few days ago, and she eats quite slowly. It seems like her meal takes at least 30 minutes. She and the rest of Team A have been returning around 12:35 or 12:40. One day, since I am in Team B and needed to eat early, I tried eating with Team A... It seems like the existing Team A members are all waiting for the new employee to finish before they move, because they feel bad leaving her alone. Now, some dissatisfaction is arising within Team B. How should I mediate this situation?
I tried to find a solution, but suggesting that the new employee goes in a later group or swapping the order of Team A and Team B feels very awkward and difficult.