Apple's software is garbage so I'm sure you're correct, but modern hard drives can do several hundred MB/s of throughput. How is that not fast enough for a freaking photo application? For Apple not to test/support this use case is inexcusable.
Scanning and accessing a photo library is extremely random I/O and has nothing to do with the peak sequential throughput. Hard drives are awful at random I/O.
If Apple kept proper indexes and thumbnails somewhere (especially somewhere fast, like on an SSD), maybe it would be fine, but I have heard some bad things about Apple Photos on hard drives, so they might not be doing things optimally.
1. https://discussions.apple.com/thread/251365140 And many, many like it.