Short description of error
Cube is moving on Y axis from frame 1 to 50. This action was pushed in NLA and I altered strip via Animated Strip Time like this: frames 1-50 use 1-15 of original action, then action idles and go rapidly to the end.
I don't understand why it is greyed out, you still can change this value and it is affecting animation quite well.
Exact steps for others to reproduce the error
Create some action, push it in NLA, enable Animated Strip Time -> greyed out playback scale