Difference in efficiency between NVL and IS NULL
To me, the logic is that the NVL value gives a specific answer to the query handler. Whereas, the IS NULL value causes an ambiguous join, thus resulting in a Cartesian product.

That is, every row on the left is trying to join with every single null on the right in the IS NULL. Conversely, in the NVL only those rows which are null on both sides join.