To reproduce:
1. Open attached project "AnimationLayerPerformanceRepro.zip"
2. Open "Profiler" window.
3. Open and Play "main" scene and open stats in Game view.
4. Press "Use Animator with layers", now each "peep_happybot" will have Animator with "Base Layer" and 11 other empty Layers. (~40 FPS)
5. Press "Use Animator with layers (weight = 0), now only the "Base Layer" will be active, other 11 layers have "weight = 0" and are not playing. (~40 FPS)
6. Press "Use Animator without layers", now there's only "Base Layer" in the Animator. (~72 FPS)

Expected result: When the weight of the layer is zero, the layer update will be skipped. Performance should be closer to the Animator without layers.
Actual result: Setting layer's weight to 0 has almost no impact (0 fps or just a few more frames increase).

Reproducible with: 5.3.6p7, 5.4.2f1, 5.5.0f3, 5.6.0b3

Note that the layers contain empty states. The performance difference between the "disabled layers" and "no layers" case increases further if the states in the disabled layers aren't empty.