Here is a patch which speed up compiling of regular expressions with big charsets.

Microbenchmark:
$ ./python -m timeit "from sre_compile import compile; r = '[%s]' % ''.join(map(chr, range(256, 2**16, 255)))"  "compile(r, 0)"

Unpatched (but with fixed issue19327): 119 msec per loop
Patched: 59.6 msec per loop

Compiling regular expressions with big charset was main cause of slowing down importing the email.message module (issue11454).