Myelinated fibres conduct action potentials faster than unmyelinated fibres.
This is because the myelin (provided by Schwann cells in the periphery and Oligodendrocytes in the Central Nervous System) has regular gaps along the length of the axon. These gaps are called Nodes of Ranvier. Instead of traveling the whole length of the neuron axon, the action potential is able to jump between the nodes. This is called saltatory propagation. This jumping is faster than the normal depolarising wave.