it is a normal logical operator which every database/programming language provides. I can understand that you might not have seen it.
It is difficult to say in theory as to how much performance it can give or it cannot give. I would say NOT is a logical operator and the other things like you should have indexes for the where clause of the query will all remain the same. So the recommendation is to test for the performance of the query in your database and fine tune if it is not giving good performance.
Thanks
