I don't really know what any of this means. You're running a single-thread graphics API (OpenGL) in a browser (does WASM support background threads?). On any modern PC, that's gonna look like 1 CPU thread doing all the work and the GPU idling 80% of the time waiting for something to happen.
yes
Figma is able to use more than 1 CPU core and also claim a lot of your GPUs capabilities.
Don't believe me? Open Figma, load a large file and watch it use multiple CPU cores using htop. Driving the GPU is not all the application does. It also has to work with a largish model and abstract syntax tree and do lots of complex things with that before it goes anywhere near the GPU and opengl. Driving that probably happens on the main thread.
On a modern M1, the app is fast and responsive. It only uses a few worker threads (less than the number of CPU cores) and the GPU is fast enough to keep framerates high if you do things like zooming and panning the view. It's the older and slower laptops that are going to be more challenging.