Performance differences using varchar, char and text

In the Postgres 7.3.3. User Guide section 5.3, it says that there's no performance difference between the three type.
  But in 'PostgreSQL Database Performance Tuning' by Jean-Paul ARGUDO (section 5: use correct datatype), it says there
is a performance difference because of you might need more I/O to read the data (which makes sense).

So, is there a performance difference?