You can mitigate some of that with web workers, but it's a shame that multiple websites can't share a python runtime the same way they can on a shared linux box. You have to download a separate one for every website that uses it. But, if you follow that train of thought to the end, you wind up with the browser itself bundling those runtimes, just like it did with Flash back in the day.

I'm not sure any of this would be an improvement over what we have now.