When you create a column on a database, and you put that column on the right of the table the column shouldn’t be added almost instantly.  However if you are assigning a default value to the column and setting the column to now allow NULL values the operation will take quite a lot longer.  This is because when the operation happens the SQL Server has to write the values to each table.

If you allow the column to have NULL values then the database doesn’t have to write the values to the column so the operation completes very quickly.

Denny