Using Adobe Photoshop Elements 2019, with 28,000 photos it took my Windows 10 Pro PC, with 16GB of memory, and an Intel I7 CPU, nearly 30 hours to work through the recognition process. When finished, I went through the photos manually and many photos had no names attached. The one that really surprised me was a passport photo of myself, with no name, even after I'd identified myself in over 500 other photos.
Facial Recognition works in a slightly different way than this. Whenever recognition runs on new images, stacks are formed in the UnNamed View and they have to be named again. What FR does is club similar faces in a single stack. So the best practice would be to let FR complete(or atleast complete by a good percentage) so that stacks are big enough and then name them in the UnNamed room.