> Python in the browser is a no-go performance-wise, of course.

"Running the Python interpreter inside a JavaScript virtual machine adds a performance penalty, but that penalty turns out to be surprisingly small — in our benchmarks, around 1x-12x slower than native on Firefox and 1x-16x slower on Chrome. Experience shows that this is very usable for interactive exploration."[1][2]

[1] https://hacks.mozilla.org/2019/04/pyodide-bringing-the-scien... [2] https://alpha.iodide.io/notebooks/300/