I was unclear the part that is a myth is that they are slow in _PostgreSQL_. COUNT( ) is actually slow in general on databases with high concurrency requirements. Because you have to sacrifice something to make COUNT( ) fast.
If you had read his benchmark you would have seen COUNT( ) was slow in both databases (three times as slow in MySQL).
If you had read his benchmark you would have seen COUNT( ) was slow in both databases (three times as slow in MySQL).