Primary Progressive Aphasia (PPA) is a rare and insidious language impairment that worsens over time. It belongs to the group of fronto-temporal dementias. This study was aimed at assessing the role of speed of cognitive abilities, such as word recognition, in PPA. The design is a single-case, longitudinal study. A male patient suffering from PPA was enrolled and fifteen healthy older adults were the control group. An event-related electrical potential connected with word recognition, namely the N400, was delayed by 200 ms at baseline compared to healthy controls and progressively deteriorated. One year later, the delay was greater and two years later the potential had disappeared. Reduced speed of processing is an early pathological factor negatively affecting higher cognitive functions in APP. Event related electrical potentials are recommended in the field of aphasia and cognitive decline. They permit observation of a speed decline in higher cognitive abilities, when pathological changes at a central level begin and language comprehension seems to be unaffected.