From my understanding - believe I read this in some SQL internals book 10 years ago - when NULL's are supported on a table then each row in that table will have a bit-pattern that indicates which columns in that ROW are null.

The added expense of processing this "is-the-column-null" bit-pattern and the extra size of the bit pattern are the performance hits of allowing NULL's.

imo - probably meaningless in the overall time used to process queries.