Well the slowed brain "rate" would effectively mean that the person would experience stimulus slower. Effectively making say a day for people outside feel like a minute to him. This would lessen the total amount of terror the person was exposed to.
However, as you say, the person has been stuck in there for over a decade, so it would still be pretty traumatic based on just the total exposure to the terrifying stimulus.
On the other hand, if you assume slower heart rate but normal brain activity, the net effect of the terror would be less. Fear is based on chemicals within the brain. With the heart rate slowed these chemicals would be distributed far more slowly resulting in less exposure to these fear related chemicals.
Effectively the near stasis effect of the carbonite should reduce the total terror of the horror of being frozen within. It will still be traumatic though. However the full effect will not be the same as say a normal person at their normal metabolic rates being immobile for over a decade.
It should be noted though, that if we assume that the brain activity is normal, the fear aspect wont be as bad as the denial of external stimulus for that duration. Imagine being unable to feel any part of your body, see anything, or hear anything and being unable to move, all you have are your thoughts. Just your own internal monologue for over a decade, will definitely result in some kind of dissociative identity disorder and probably severe schizophrenia and delusions. Having lived for so long without any stimulus, the person in question will be unable to deal with the sudden renewal of outside stimulus and would be overwhelmed, or unable to discern reality from the thoughts of his own mind.
One last note. The bodies natural reaction to these kinds of traumas are unconsciousness and coma. The brain effectively shuts down to spare itself from facing the realities of the situation. Being trapped but aware, would quickly result in the person lapsing into a coma to deal with the trauma.
Of course reality and science are not as much fun, and star wars is pretty much the poster child for science ignoring science fiction, so Go with whatever seems the most fun for you.