Photos will continue scanning your remaining X photos...How long and why so long?

Sorry if this has been answered somewhere but can't actually find an explanation as to why this process should take days/weeks/months even if computer left plugged in, not used for anything else and photos either minimised or closed. Can't find any response from Apple anywhere either as to why this should be so.

I have about 11K photos and very few videos, in fact the majority are filmed on the iPhone. I checked the couple of videos that are from other sources and they opened and even tried removing them from the library but that doesn't seem to have made any difference. Running Catalina and all software up to date and all photos stored locally on Mac.

Scanning was completed when I first upgraded to Catalina (after similar problems) but has started again. I can just ignore it but when I go to my People Album and look for certain people all I can see is "Updating XX..." and cannot view album.