[Mecanim] Delay between calling play and actual playback

I got an animation controller with 4 layers, NO transitions (so no "HasExitTime"), and a float parameter which is 1 or -1.The parameter (="SpeedMod") is used as Speed Multiplier in some states to determinate the direction of the clip. Since my timescale is 0 the animation mode is "Unscaled time". First I use the runtime controller to setup events so that a method is invoked after a animation ends. SpeedMod is set to 1 at beginning.
Then I do the following:
1)Play a state with MyAnimator.Play ("Zoom", MyAnimator.GetLayerIndex ("Position"));
2)After that is done I wait for the user to Click a button.
3)Set SpeedMod to -1
4)Play the same state again with MyAnimator.Play ("Zoom", MyAnimator.GetLayerIndex ("Position"));

Everything worked fine, but it seems since I updated to Unity 5.3.4 I encounter a problem:
After 4) there is a massive unwanted delay (up to 30 secounds) between calling the Play method of the animator and the actuall playback starts.

I have no idea why.