> While "40x faster" is more 10x faster than "4x faster", C 
> implementation can boost only CPython and makes maintenance more harder.

As a counter argument against "let's not do it because it'll be harder to maintain" I'd like to point out that namedtuple API is already kind of over engineered (see: "verbose", "rename", "module" and "_source") and as such it seems likely it will remain pretty much the same in the future. So why not treat namedtuple like any other basic data structure, boost its internal implementation and simply use the existing unit tests to make sure there are no regressions? It seems the same barrier does not apply to tuples, lists and sets.

> Of course, 1.9x faster attribute access (http://bugs.python.org/issue28638#msg298499) is attractive.

It is indeed and it makes a huge difference in situations like busy loops. E.g. in case of asyncio 1.9x faster literally means being able to serve twice the number of reqs/sec:
https://github.com/python/cpython/blob/3e2ad8ec61a322370a6fbdfb2209cf74546f5e08/Lib/asyncio/selector_events.py#L523