The conduction of the action potential (AP) down a myelinated axon is very fast because

under myelin, the AP travels like electricity in a wire, which is much faster than it travels along axon membrane without myelin

the extra sodium present in saltatory conduction speeds up the AP

the AP becomes subthreshold under myelin, and subthreshold potentials travel faster

without myelin, the AP 'leaks' to the axons of other neurons and this leaking process slows down the AP