I do not think this is unreasonably slow. I recently decided to clear out all my old test catalogues and start again, with a single new catalogue of all my images, nearly 700,000 of them. The catalogue is on a SSD drive and all the images are on local hard disks. I am on Windows 10 on a well-specified modern desktop computer. Building the catalogue took about 4 days, but given the sheer quantity of data and previews to be created, I thought that this was not unreasonable. I have another DAM, which took much longer to complete a similar task (and is much slower to use having completed it).
The only issue I had was that the database appeared not to include fully a number of files. Metadata import appeared to be stuck and images were appearing as offline. I do not rename my images, the file names remain as out of camera. That means that I have many files with identical file names, spread over my various annual folders (I keep images in folders by year, then sub-folders by location or subject) spread over several drives. For files marked as offline, sorting by date did not separate them from their similarly named counterparts - they appeared grouped together by name in the contact sheet, even though differently dated. I concluded that the problem was the database trying to catalogue simultaneous identical file names in parallel (but only a guess, I am no expert). This was before the recent release which allows a search for offline files, but I could see that most of the apparently offline files were in the most recent folders. I therefore opened a contact sheet to load all my most recent folders and sub-folders, to force PM to "see" them and that they were online, not confused by identical file names from older folders. This worked fine, and all the metadata updated quickly, and all images were recognised as being online.
I have not seen this behaviour reported elsewhere (but I may have missed it). If anyone else has had similar issues, possibly caused by having a lot of identically named files being indexed at the same time, resulting in apparently offline images or stuck metadata, this worked for me, and I hope this helps. For comparison, creating the catalogue for only my 2020 images, about 45,000, completed fully in about 2 hours. Perhaps it might be better to drip feed folders into the catalogue rather than select the whole set of several hundred thousand across several drives at the same time for a single addition on creation of the catalogue.
Graham