neuroscience - Saltatory conduction of nerve impulses

I am aware about some basics of saltatory conduction of nerve impulses. I know that the nerve impulses (ion flow and the depolarization) are transferred from node to node in myelinated nerve fibers. In non-myelinated fibers, the ion flow and depolarization are repeated along the entire length of the axon. Thus the saltatory conduction is faster.

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Why is it so that myelinated nerve fibers require less energy of activation then the non-myelinated ones?

Also, my book says that a rise in temperature accelerates the conduction of nerve impulses. I would like to know the reason for this.