Calculate FPS in MJPEG Stream (HTTP multipart/x-mixed-replace)

By definition it's Frames per second, but how can I reliably identify frames and time received on the client side? On the backend, I can set timers on each frame being sent, meanwhile on the browser it does not create multiple requests, but perhaps one stream of Content-Download response rewriting each image (rendered using <img/> tag). My goal is to measure fps as a metric of the whole end-to-end streaming pipeline. Is there any third-party javascript library perhaps that can calculate each image rendering and can measure the time between frames rendered? How do I go about this, is this even possible?