This uses Pyodide [0] under the hood [1], which is CPython compiled to WebAssembly. In all my tests of it, loading takes a long time ~5 seconds. Coldbrew [2], another distribution of CPython on Wasm, is another option with similar load times.
And Brython [3] is a completely different option without long load time: a Python interpreter implemented in JavaScript.
If load time is important, Brython is pretty nice. If feature completeness is important, Pyodide and Coldbrew are probably best.
There are actually quite a few Python-in-the-browser implementations, not just those two! A couple of years ago a colleague of mine wrote up a comparison, including Brython and Pyodide:
(We chose one of the Python-to-JS compilers, https://skulpt.org, for the Anvil web-app platform, because it's much lighter weight than this. It's a couple of hundred kb on the wire, and you can call to the server for any heavy lifting.)
https://anvil.works/blog/python-in-the-browser-talk
(We chose one of the Python-to-JS compilers, https://skulpt.org, for the Anvil web-app platform, because it's much lighter weight than this. It's a couple of hundred kb on the wire, and you can call to the server for any heavy lifting.)