Just did some benchmarks, and while Signature has apparently had a big speedup in Python 3.6, it is still much less performant than either the Python 2.7 or Python 3.3 implementations, anywhere from 6-18 times slower approximately depending on the function.   For those of us that need a getargspec that is only used on selected, internal functions where we don't need the newer language features, it's likely worth it to vendor the Python 3.3 getfullargspec() implementation which is very simple:

https://gist.github.com/zzzeek/0eb0636fa3917f36ffd887d9f765c208