I recently migrated around ~50,000 photos and videos from Aperture to Photos (see my blog post on the process), and have also in a short amount of time upgraded my personal and work Mac laptops (both from older MacBook Airs to newer MacBook Pros).
On both of my new laptops—which were at least 3x faster than my older Airs—I noticed that Photos started completely fresh in its photo analysis for the 'People' album that shows everyone's faces. And after three weeks of seeing one of my CPUs stick around 100% all day every day (while plugged in), I started getting sick of this.
I would leave the Mac on all night, and check in the morning, and only 20-30 new faces would be recognized.
Some days it seemed it would take forever...