How can I make the face recognition process continue? Right now, if I go on the People section of photos.app, a message says it has analysed x photos, while the remaining y photos will be analysed when I'm not using the app and my Mac is connected to a power source.

What exatcly is meant by "not using the app"? Should it be open, but not the foreground application? Hidden? Closed? Can it run while the Mac is in sleeping? (I saw in the logs photoanalysisd   asking to run at priority Maintenance).

I'm asking because it's not making the progress I would expect andI'm having trouble understanding exactly how should I leave it.
According to this blog post Photos.app should be closed, as photoanalysisd is suspended while the application is open.
The explanation given in the Photos application is definitely too vague. Still, I have the same issue of not experiencing the progress that I would have expected (4.900 / 12.000 in seven days).