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How can I turn off light emitting materials so they don't affect render times?

Is it enough to turn the power to zero?

A quick test appears to indicate that turning the power to zero is not enough -- in this test, performance is about the same regardless of wattage, but nearly doubles when switching the material to a different type (that is the answer to your first question). I guess this would be highly dependent upon the actual scene, though -- this one is Susan standing on a plane, with 96 spherical emitters above, and skydome intensity set to 10 cd/m2, since without it, we would get a "no emitters in the scene" error when the emitter was swapped out.