Silence is completely acceptable when it is purposeful. Interviewers would generally rather see you take a short pause and return with a clear answer than listen to unstructured thinking while you search for one.

Tell the interviewer what you are doing: 'Let me take a few seconds to structure that.' Then use the pause to organize the answer rather than apologizing for the silence.

Twenty or thirty seconds can feel long to you but is often fine for a real thinking task. The bigger issue is taking the time and still returning without a clear structure. If useful, this guide on structured communication in case interviews gives more detail.