All questions are good, as you will see!
OK, I was just going to respond and although it did stay at 30% for a bit, it was still fairly quick.
I also tried saving via Collect Clip, which brings up a dialog where you can select where save the motion. That seems to go markedly quicker so you could try that.
In the Collect Clip track, select the time stretch of the clip you want to save. Right-click and then select Add Motion to Library.. A dialog will then let you select where to save.
Investigating further, I found what happened: when adding the clip with the "+" sign, the whole timeline is saved, irrespective of the length of the clip
. So in your case, shortening the clip didn't make a difference. When using Collect Clip it will only save the time stretch that is selected.
Another option is to set the project length to the length of the clip you want to save. So if it's 300 frames that is what you would set the project length to. Note that shortening the active time stretch on the timeline with the green markers has no effect: you need to set the project length. I think using Collect Clip is a bit easier...
So because of your question. I learned something today.