Face recognition is possible on a tiny and cheap ESP32-CAM[0] using Espressif's own ESP-WHO[1] framwork. Apparently it can process images at around 3 fps. I have an ESP32-CAM on order to try this out as part of a side project of mine[2], so I can make the eyes look at faces :) I would also love to get hold of a Person Sensor[3] which runs custom firmware specificially designed for face detection and would be absolutely perfect for this, but they're not in stock currently.

There's other libraries[4][5][6] that look like they could work too and I plan to try. As far as microcontrollers goes, the Teensy 4.0[7] should be powerful enough for reasonably fast processing (and it has a floating point unit!), though I'm yet to find a good library for doing so.

[0] https://robotzero.one/face-tracking-esp32-cam/

[1] https://github.com/espressif/esp-who

[2] https://github.com/chrismiller/TeensyEyes

[3] https://usefulsensors.com/person-sensor/

[4] https://github.com/ezelioli/Face-Detection-on-Microcontrolle...

[5] https://github.com/nenadmarkus/pico

[6] https://www.reddit.com/r/embedded/comments/g77exq/recommende...

[7] https://www.pjrc.com/store/teensy40.html