There are websites like https://pimeyes.com or http://faceagle.com that can search for a face images. And they are really fast, How this magic works?
I'm using Elasticsearch to store and imdex face embeddings data... It takes aprox. 5 sec to find out 70% similar face among 10 millions others on my hardware. In fact, the search engines compare not faces but their encodings / embeddings; the last ones are N-dimensional (for exapmle, 128d) vectors. The most popular mathematics for such comparasion are Euclidian or cosine similarity of the vectors.