A Spell Singer is a natural singer, able to find the natural rhythm and beat of existence. If they know magic (and most of them do), they'll invoke spells through song (or at least chants) and possibly rhyme, rather than through sheer force of will and arm-flailing.

"Spellsongs" (as they are known in Man's lore) take fifty percent longer to cast than their less-musical equivalents, but generate fifty percent less Fatigue for the caster. Note that the gem cost is the same. For example, say a Mother of Avalon with 2 spends 1 to cast Swarm; this costs her only 51 Fatigue [(100 + Spellcasting Encumbrance) / 2]. If a Pangaean 2 Dryad spent 1 and prepared to cast Swarm on the same turn, they would pass out from the full 103 Fatigue cost, but their Swarm would definitely arrive first.

Are mutes still spell-singers?