You had lots of time to think during your PhD. You had plenty during your prep. You have enough in the viva.
Your examiners want you to consider, ponder, debate, reflect, examine, muse, propose – to think! – and then respond.
This doesn’t mean long silences, longer vivas or almost-impossible questions. Just enough time to think about something important.
Take time in your viva. It’s there for you to use.