WASM gave Figma a lot of speed by default for a lot of perf-sensitive code like rendering, layouts, applying styles and materializing component instances, our GUI code is mostly React and CSS.
WASM engine performance has not been a problem for us, instead we are constantly looking forward improvements in the devex department: debugging, profiling and modularization.
One of the largest challenges of the platform we face today is the heap size limit. While Chrome supports up to 4GB today, that's not yet the case for all browsers. And even with that, we are still discovering bugs in the toolchain (see this recent issue filed by one of our engineers) https://github.com/emscripten-core/emscripten/issues/20137
The challenge of the perf-testing at scale in our company is helping developers to detect perf regressions when they don't expect them - accidental algorithmic errors, misused caches, over-rendering React components, dangerously inefficient CSS directives, etc.
Would love to see that cap get raised across the board, it'd enable us and others to do so much more.