Microbenchmarking hash computation (Linux, gcc 4.7.3):

* Short strings:
  python -m timeit -s "b=b'x'*20" "hash(memoryview(b))"

- 64-bit build, before: 0.263 usec per loop
- 64-bit build, after: 0.263 usec per loop

- 32-bit build, before: 0.303 usec per loop
- 32-bit build, after: 0.358 usec per loop

* Long strings:
  python -m timeit -s "b=b'x'*1000" "hash(memoryview(b))"

- 64-bit build, before: 1.56 usec per loop
- 64-bit build, after: 1.03 usec per loop

- 32-bit build, before: 1.61 usec per loop
- 32-bit build, after: 2.46 usec per loop

Overall, performance looks fine to me.