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As aek and dahart said, updating a BVH (refitting) is not expensive. If anything, it's bandwidth limited.

The challenge is minimizing the cost of (selectively) rebuilding.

The method from the paper safes both, memory bandwidth and compute time. You animate and write (to memory) a way smaller number of vertices. You update a smaller (less memory bandwidth, less compute) BVH.

That’s just another way of saying “updating a BVH is expensive therefore spend effort to minimize the amount of updating it”.

The fastest code is the code that never runs.

There’s a material algorithmic difference between rebuild and refit. Not sure if that point was clearly understood.

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