> So we need a code path that is specifically for calling the str() function on ints. I would argue that this is prohibitively specialized for an interpreter,

I would argue it isn't. It's actually less about the ints but about the tuple allocation:

"Now that we've removed enough code we can finally make another big optimization: no longer allocating the args tuple This cuts runtime down to 1.15s." (down from 1.40s)

It seems to me that having special case for one argument, avoiding the tuple allocation in each call is nothing "prohibitively expensive" and would benefit all the functions being called with one argument, and there are enough of such.

Regarding ints vs something else -- somewhere in the code there is anyway different code that does str of int vs str of something else. It's just about accessing that code for that type, it doesn't have to cost having today's speculative execution in the CPU's. The people who produce compiled code know about that and can use it.