CROP doesn't have much to do with speed.
Here's the initial Change Clip Speed dialog for a :30 clip... 
Change the Duration here - in this case to :15 - and the speed will track... 
Similarly, when a clip is sized on the timeline, the Speed percentage that you have entered in the Speed Change box will change accordingly.
Perhaps not precisely what you want, but something to be aware of. Click the caret at the top-right of the VP window and make a suggestion.