Evokers carry a Totem of Undying... so why don't they use it?

They do now! When you kill an Evoker, he will activate his Totem of Undying to save himself. The Evoker does not consume the totem as players do, because it's his totem and he knows how to untap its full power. He rescues himself from death in the same manner players do, with one half a heart of life, absorption, fire resistance, and regeneration. While he is in this enchanted state, he is vulnerable, since he can't use the totem again right away. Once he achieves his full health, and his enchantments wear off, his totem will be ready to use again, so don't wait to dispatch him! Only while he’s recovering will he perish for good, and drop his valued Totem of Undying for players to acquire.

Zero per-tick functions, so zero drag and lag on your busy server.