I'm not going to discuss pathophysiology of Parkinson's Disease (PD) in details here, but I'd like to share my experience of working with people with PD.
Bradykinesia (slow movement) and Akinesia (muscle rigidity) are common motor symptoms of the disease. The disease slowly affects movement quality and posture over time. As people start to move more slowly with smaller range of motion, the nervous system starts to adjust its sensory awareness/perception as the movement changes. Because this change continues gradually, people may not notice such change, even though it may be apparent to other people's eyes. What will happen is that their sensory awareness becomes so distorted that they perceive their abnormally slow and small movements as being "normal." When they see other people moving at their regular speed, they don't perceive them as moving faster with larger range of motion than they are. It's like a thermostat that is off.
PD is often considered as a movement disorder, but to be more precise and accurate, it is a sensory-motor disorder. Movement and sensation are constantly influencing each other and being updated in the brain. Areas that you don't use much tend to have poor sensory awareness. Areas that you use very frequently tend to have good sensory awareness. How accurately can you sense specific parts of your low back without touching and seeing? How accurately can you sense your individual fingers? It's much more clear to sense your fingers, correct? This is very important when working with people with PD because without changing their sensory-motor awareness/perception (calibrating their thermostat), they are much less likely to change their movement quality because their inaccurate perception tells them they are moving just "normal."