A difference to Python is that Python has the escape hatch with native modules. A PyQt application as arbitrary example relies on native code for doing most of

A difference to Python is that Python has the escape hatch with native modules. A PyQt application as arbitrary example relies on native code for doing most of the handling and only some small parts of business logic have to be done in Python. With WebAssembly the Qt stuff is compiled to WebAssembly as well and therefore suffers from the slowdown as well. Only the actual rendering can be offloaded to WebGL and hardware. (Still we were able to write snappy UIs more than a decade ago, where systems were a lot slower so for many cases that should be fine)