Turbofan optimisations, SharedArrayBuffer, Web Assembly really impacted performance of my game engine. With enough assets it's already possible to make GTA V complexity and quality game running in the browser. The biggest progress in browsers and webgl performance happened in the last 3 years so it doesn't matter what you've been hearing 10 years ago. It already happened and is possible NOW, even on phones.
"Loading time is latency too" no it isn't. It doesn't matter if you're getting closer to an object and in the distance it fades in(or loads higher LOD) even with 500ms delay. But in streaming game as a video when the whole gameplay is delayed by 300ms it already makes entire competitive game unplayable.