It's fast on the backend due to running on the JVM and being completely non-blocking (Scala + Akka + MongoDB)
Combined with a lightweight-ish frontend (Cordova build for native mobile) and, voila, impressive performance with a decent UI all while not having to maintain separate web, iOS, and Android apps.
Am I mistaken, or was it that lichess was actually in PHP a couple of years ago? I seem to remember that I was surprised to learn that it is Laravel+Vue. But maybe I'm mixing things up.
Combined with a lightweight-ish frontend (Cordova build for native mobile) and, voila, impressive performance with a decent UI all while not having to maintain separate web, iOS, and Android apps.