I've heard the statement that Python would be too slow to be of any use in browsers.
I reckon Javascript is only superior in this aspect because of companies like Google who need it fast (and made it fast) because they need it to survive, but I could be wrong.
Are there any differences in how Python and Javascript are designed that have an impact on how they (would) perform in browsers?
Since as of now there isn't a client side Python implementation, my question comes from the statement someone made, so maybe it has something to do with the languages itself (although I don’t believe that).
Best Answer
To start with, we must make a clear distinction between languages and implementations. A language is an abstract thing, the implementation is a concrete thing that can have performance measured. For example, Lisp was once considered far too inefficient for practical use but compilers kept maturing and, eventually, dedicated hardware was developed for it; at one point in the 1980s it was the development platform of choice for high performance workstation development.
That said, the simplest answer is that a fast Javascript implementation like Google's V8 blows the standard implementation of Python (CPython) out of the water. V8 is a highly optimized virtual machine with a JITer that is amazingly fast while CPython is a fairly simple VM in comparison. There's an implementation of Python with a JIT but that is still only about 5-6x faster.
Five years ago it would have been a different story. Browsers had simplistic Javascript implementations because speed wasn't a concern since nobody built 'real' software with it and Python would have been equal, if not faster.