[coder543]

Modern external hard drives are more like 120MB/s, not "several hundred" MB/s, last I checked, but that's only for sequential access.

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.

[compiler-guy]

The pain many people experience with using external hard drives and Apple photos is photos building that index and those thumbnails. Huge amounts of io that apparently need to be rebuilt constantly [1]. Probably also happens on SSDs, but you can’t tell because you can’t hear.

1. https://discussions.apple.com/thread/251365140 And many, many like it.