I know, but i figure people will read his comments first and then see this.
I'm not particularly concerned about the standard library, but about the more dynamic features of the language. Not just exec/eval, but for example the complex dispatching logic behind magic methods, especially the various __getattr__, __getattribute__.
They are where alternative runtime implementations usually stumble, as if they were an intrinsic bottleneck to Python runtime performance.
exec/eval do not work, but all of the other dispatching and magic methods are supposed to work exactly like CPython (and if they don't, it's a bug/not yet implemented).