A nerve impulse in a healthy nerve can travel at speeds close to 120 meters per second. This speed is achieved because the nerve impulse bounces from one Node of Ranvier to the next, since the axon inbetween these nodes is insulated by myelin.
In demyelination disorders this speed is severely compromised. For example in Multiple Sclerosis which also an autoimmune demyelination disorder but affecting the central nervous system, this speed of conduction can be as low as 8 meters per second. Guillian-Barré Syndrome affects the peripheral nervous system and is known to completely block nerve conduction and where conduction continues it is generally below 20 meters per second.