Hello again.
I'm sorry you're having the issue with face tagging of large libraries/folders.
Here, I've tried to replicate your circumstance by importing folders with "solid" numbers of photos, many of which include some of the same faces tagged previously (i.e. the same people).
The current project now has a total of 2000+ photos, which I've face-tagged progressively as each folder is imported. During that process, I've not noticed any slowing down.
Folder 1 ~100 photos - 0:58 | Folder 2 ~250 photos - 1:23 | Folder 3 ~360 photos - 1:56 | Folder 4 ~ 200 photos - 1:17 | Folder 5 ~ 420 photos - 2:15 | Folder 6 ~180 photos - 1:08 | Folder 7 ~ 260 photos - 1:25 | Folder 8 ~ 300 photos - 1:38 (times according to face tag progress bar)
I should add that, right now, the PC I'm using is in need of some maintenance & is not running as well as usual. That aside, I cannot suggest a possible cause for the slowing down you're experiencing.
Certainly, I can see that if one used PhotoDirector as a complete library of ALL photos, the database of face tags etc would become bloated. For this 2000+ project alone, the face tagging cache folder is >2GB.
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