emscripten ships a "desktop GL" emulation library [0], which can have quite a bit of overhead. If you want something faster, you can use the native WebGL bindings [1]

[0] https://github.com/emscripten-core/emscripten/blob/main/src/... [1] https://github.com/emscripten-core/emscripten/blob/main/test...