Yes, the backgound processes (object and face recognition, curation of the layout) will have to run again.
Photos is not yet up to handle so large libraries, even on a very fast hardware, at least not, if we are not patient and not willing to wait a month or so for the background scanning processes to get the work done. The artificial intelligence used in Photos is very powerful, but needs a lot of processing power. It is great for libraries with just a few thousand pictures, but for very large libraries we have to be very, very patient.
My solution is to keep my iCloud Photos Library small and my archive of all photos on external volumes, not in icloud.
Only my Favorites ind the photos I am currently working on are in iCloud Photos and appearing on all devices. This is reducing the amount of background processing, when I need to repair or move the iCloud Photos Library, and I can still access all important photos on all devices, and they are easier to find on my iPhone in a small library .
Instead of archiving all photos as a gigantic iCloud Photos Library in iCloud, I am saving all original images files on iCloud Drive by exporting them to folders named like the Moments for easy access. This way I am having an off-site backup in iCloud of the originals as well.