Ondrej Zizka#DigiKam's face matching quality AND drops terribly as the photos collection grows. With decades of photos of a single family, faces match almost randomly, and confirming a face takes 15s on a high-end machine.
This is due to the KD-tree used for the index and lack of using the modern vector features of MariaDB / SQLite.
Hopefully, some near version will switch to multi-cluster centroid-based matching, speeding things up by orders of magnitude.