You are right for the most part. I attended a talk about pyscript[1] (runs python in the browser using wasm which is similar) and there is a 2x performance hit.
PyScript has actually been massively refactored in the last few months and is much faster now. You can check the performance of PyScript running MicroPython here: https://laffra.github.io/ltk/
If you need/choose Pyodide load time will of course by much more but that's mostly because of the size of full core Python and the dependencies you might have.
[1] https://pyscript.net