Sounds like they are doing it wrong.
Our Wasm investigations started in August 2020, when we built some prototypes to compare the performance of Wasm VMs and JavaScript VMs in simulations involving the type of work our low-level JavaScript components were doing. In those experiments, code written in Rust and compiled to Wasm was 10 to 25 times as fast as JavaScript.
For video processing, especially high fidelity, high frequency, and high resolution video, I can see WASM crushing JavaScript performance by orders of magnitude. But, that isn’t this. They are just launching an app.
I have verified in my own personal application that I can achieve superior performance and response times in a GUI compared to nearly identical interfaces provided by the OS on the desktop.
There are some caveats though.
First, rendering in the browser is offloaded to the GPU so performance improvements attributed to interfaces in the browser are largely a reflection of proper hardware configurations on new hardware. The better the hardware the better a browser interface can perform compared to a desktop equivalent and I suspect the inverse to also be true.
Second, performance improvements in the browser apply only up to a threshold. In my performance testing on my best hardware that threshold is somewhere between 30000 to 50000 nodes rendered from a single instance. I am not a hardware guy but I suspect this could be due to a combination of JavaScript being single threaded and memory allocation designed for speed in a garbage collected logical VM opposed to allocated for efficiency.
Third, the developers actually have to know what they are doing. This is the most important factor for performance and all the hardware improvements in the world won’t compensate. There are two APIs to render any interface in the browser: canvas and the DOM. Each have different limitations. The primary interface is the DOM which is more memory intense but requires far less from the CPU/GPU, so the DOM can scale to a higher quantity of nodes without breaking a sweat but without cool stuff like animation.
There are only a few ways to modify the DOM. Most performance variations come from reading the DOM. In most cases, but not all, the fastest access comes from the old static methods like getElementById, getElementsByTagName, and getElementsByClassName. Other access approaches are faster only when there is not a static method equivalent, such as querying elements by attribute.
The most common and preferred means of DOM access are querySelectors which are incredibly slow. The performance difference can be as large as 250,000x in Firefox. Modern frameworks tend to make this slower by supplying additional layers of abstraction and unnecessarily executing querySelectors with unnecessary repetition.