I am not sure that I understand. Does the adjustment you make to the speed graph disappear or are you undoing the adjustment. My speed graph editing works as expected. Your movie looks normal until the graph goes back to the normal position.
Maybe you are referring to the auto scaling of the graph as you drag. If so, this is normal behavior.
Maybe more clearly in this video. It does an infinite sliding down.
This didn't happened to me in previous versions.
The graph editor has done that for me on a Mac since it was introduced. Just checked CS5, CS6, CC and CC2014... All identical. The only hitch I've ever seen is when AutoZoom Height is unchecked. This can slow down but not foul up the workflow.
As long as your adjustments remain in place after you release the click then the Graph Editor is working as expected.