If someone is unconscious, but breathing, do you need to perform cpr?

No. If they have a pulse and are breathing, there is no need for cardiopulmonary resuscitation (CPR) or chest compressions. Unconscious people can have both, one or none of the above. In the new guidelines for cpr, if you come across someone who is unconscious and not breathing, start doing rapid chest compressions. Trained medical professionals are encouraged to look for a pulse before starting cpr.
No. CPR is reserved for people with cardiac arrest. These people stop breathing and have no heart beat? Without CPR and rapid emergency care death is 100%. As a bystander, call paramedics, start CPR and use an AED if available. All adults and adolescents should be CPR certified.