30s to load Loupe View image (d850)

That is indeed ridiculously long. To see an image in loupe view, Lightroom needs to load the preview. That's where the problem might be. Try the following: in your catalog folder there is a file called ‘catalogname Previews.lrdata'. Quit Lightroom, move this file out of the folder and start Lightroom again. Lightroom will now rebuild the previews, so initially you will see empty thumbnails, but they will reappear one by one. See if that solves the problem of going to loupe view.