As I said, WebGL API is single threaded, and can only be used from a single thread. Off-screen canvas (OffscreenCanvas - Web APIs | MDN) allows it to be used from a worker thread, but then all rendering moves to that thread. It does not allow you to render from multiple threads in parallel.
So this typically helps if you use html webpage, and WebGL is rendering on separate thread, that can work without blocking the main webpage.
At the moment, Playcavas does not support this.