Pyodide (standard cPython in WebAssembly) loads surprisingly quickly.

My https://lite.datasette.io application usually starts up in less than 10s - most of that is downloading about 10MB of WASM blobs, and 10MB isn't actually that big these days (many sites serve more than that in image headers).

When I built Datasette Lite I did it as a research project, assuming it would be far too slow loading to be useful. I've since changed my mind on that.