There is some well-informed speculation about latency up-thread, but I've tinkered with OpenCV from time to time. Specifically, I used ROS and OpenCV to do face detection on a netbook in about 2012. That worked out pretty well - it would track multiple faces in realtime. Edge detection is a different algorithm, but isn't super likely to be the origin of the latency issues they're having. It's just a threshold check over a window of a few (e.g. 3 or 5) pixels, multiplied by the dimensions of the image.