[jzwinck]

Faster than fastest:

  import numba
  @numba.vectorize(nopython=True)
  def add2(x, y):
      return x + 2 * y

  add2(X, Y, out=X)

This is 40% faster on my machine. It only needs to read X and Y once, and write X once. It does take a bit of extra time on the first run, because it uses JIT compilation (LLVM).

Numba lets you implement vectorized operations in terms of elements. In the above, x and y are scalars. You get support for various NumPy "ufunc" features for free, such as the out parameter used here.

Ref: http://numba.pydata.org/numba-doc/dev/user/vectorize.html