Charles-François: you get the idna overhead in 2.7, too, by specifying u'127.0.0.1' as the address.

The idna overhead could be bypassed fairly easily in C by:
1. checking that the string is an ASCII string (this is possible in constant time, in 3.x)
2. directly passing the ASCII string to setipaddr (leaving any error detection to this routine)

Before adding caching, I'd check whether a cache lookup is actually faster than calling inet_pton.