$ ./python -m timeit -s 'import re; s = "a"' 're.sub("(a)", r"\1", s)'
100000 loops, best of 5: 2.45 usec per loop
500000 loops, best of 5: 860 nsec per loop

$ ./python -m timeit -s 'import re; s = "a"; p = re.compile("(a)")' 'p.sub(r"\1", s)'
200000 loops, best of 5: 1.79 usec per loop
500000 loops, best of 5: 546 nsec per loop

$ ./python -m timeit -s 'import re; s = "a"*10**3' 're.sub("(a)", r"\1", s)'
500 loops, best of 5: 620 usec per loop
1000 loops, best of 5: 252 usec per loop

$ ./python -m timeit -s 'import re; s = "a"' 're.sub("(a)", r"b", s)'
500000 loops, best of 5: 711 nsec per loop
500000 loops, best of 5: 663 nsec per loop

$ ./python -m timeit -s 'import re; s = "a"' 're.sub("(a)", r"\n", s)'
200000 loops, best of 5: 1.7 usec per loop
500000 loops, best of 5: 864 nsec per loop