On second thoughts I decided to try it now 
It still seems to be struggling - for 3 minutes now it’s been sitting at 94% (with the same big pano I posted above). CPU usage is not 100% but I haven’t got to the point where I crtl-clicked images last time.
I will try with a smaller section next.
Right, 7 images vertically worked perfectly and only took about 15 sec or so (I wasn’t timing…) to get to the preview:
… but 15 or so pics, (two vertical strips) seems to have pushed it over the edge again, in terms of responsiveness - been sitting at 94% for a couple of minutes with no sign of life 