Sebastian - I miss-wrote: the beta version I was talking about installing was of the RTF/ODF scan add-on. But I did indeed still run the beta version of Zotero itself too. I have now installed the final version of Zotero 5 and have removed and re-installed the beta of the RTF/ODF scan add-on, but I still get the same problem (here is where I see that it takes so long - I'd say about 2 full minutes or so). Which is ironic, because my actual (very large) libraries now do indeed load a lot faster :)
So the add-on does appear to install properly (see here); I did download and put the scannable cite in the right folder (see here), but I cannot see it in the list - even after closing and re-opening Zotero (see here). Any ideas about what might have gone wrong? Thanks!