What is the fastest way to get the reverse complement of a DNA sequence in python?
I don't know if it's the fastest, but the following provides an approximately 10x speed up over your functions:

import string
tab = string.maketrans("ACTG", "TGAC")

def reverse_complement_table(seq):
    return seq.translate(tab)[::-1]


The thing with hashing is that it adds a good bit of overhead for a replacement set this small.

For what it's worth, I added that to your code as "with a translation table" and here is what I got on my workstation:

global dict implementation
       1.37599s total,
       363374.8 strings per second
       3.3% increase over baseline
naive implementation
       1.44126s total,
       346919.4 strings per second
       -1.3% increase over baseline
with a translation table
       0.16780s total,
       2979755.6 strings per second
       88.2% increase over baseline


If you need python 3 rather than python 2, then substitute tab = str.maketrans("ACTG", "TGAC") for tab = string.maketrans("ACTG", "TGAC"), since maketrans is now a static method on the str type.

For those wondering, using biopython is slower for this (~50% slower than the naive implementation), presumably due to the overhead of converting the strings to Seq objects. If one were already reading sequences in using biopython, though, I wouldn't be surprised if the performance was much different.