There are all sorts of "slowing" that can show up on an EEG. Whether it's abnormal or not depends a little bit on the age of the person and other external factors -- but generally, slowing IS considered an abnormality. Keep in mind that it's one of the most common EEG findings. Slowing can suggest that there's a problem, but not what the specific problem is. Some possible causes: migraine, seizure, stroke, hypoglycemia or hyperglycemia.
When the neurologist looks at the slowing, the main factor they consider is the speed of the wave. The slower it is, the more serious a problem it indicates. If there are continuous bursts of slowing, epilepsy might be a likely cause. If the slow waves speed up and slow down over and over, but don't speed up to normal during that episode of slowing then epilepsy is also likely. And if the slowing is focused in the temporal lobe, that's another epilepsy indicator.
Hope this helps!