INADA Naoki: "I want to Python 3.4 and Python 3.5 solve this issue since it's critical problem for some people."

On microbenchmarks, the optimization that I just implemented in Python 3.6 are impressive. The problem is that the implementation is quite complex. If I understood correctly, you are asking to optimize decoders and encoders for ASCII and UTF-8 (modify 4 functions) for the surrogateescape error handler. Is that right? Would UTF-8 be enough or not?

I don't like backporting optimizations which are not well tested right now. To optimize encoders, I wrote a full new _PyBytesWriter API. We cannot backport this new API, even if it's private. So the backport may be more complex than the code in the default branch.