Viva prep is less.
Less reading than when you were researching.
Less writing than when you were writing up.
Less thinking than when you were figuring things out.
The viva is a fraction of the time you’ll spend on getting ready, and viva prep is a vanishingly small period compared to the months you’ll put into your PhD. Perspective matters. You need to get ready for the viva, but the real work that helps you pass has already been done.