Yes, Python is among the slower interpreted languages out there, but still. The debate between fast execution and programmer productivity is not a new one. Given Python's popularity, I'd say it's settled.
If the user experience is significantly worse, I don't care if it was easier for you to write the code.
Pyodide runs at least ~10MB. That's a lot for a web app to handle, for sure. The use case there is most likely either scientific computing/data science (where JavaScript is just an obvious non-starter), or it's line of business somewhere where someone downloads an ERP app and just runs it. Those are already huge, as anyone who uses the stuff from ERP companies can attest.
No one is seriously proposing that you should use Pyodide, with the whole Python standard library, to add interactivity to a regular newspaper website or something like that.
MicroPython might be a good fit for that though. It's ~100k -- smaller than a lot of JS tooling.
But usually when we say "fast language" or "slow language," we're talking execution speed, not runtime size.