Enabling "fast mode" for the Tonemapping Color Grading component makes it run slower (Fast mode is enabled as default).
In my test: 0.51 - 2.82ms with, 0.02 - 0.14ms without.

Repro Steps:

1) Open attached project and scene "RealTMTest"
2) Enter play mode
3) Open profiler and look at ms results for TonemappingColorGrading (under CPU > Camera.Render > Camera.ImageEffects > Camera.ImageEffects)
4) Exit play mode and change line #17 in "TonemappingColorGrading.cs" to true
5) Repeat steps 2 & 3 and notice performance decrease

Workaround: Disable fast mode (appears to offer no advantage)