Performance degradation of .ix from in pandas 0.13.1
I see a performance degradation with the following code between 0.12 and 0.13.1:
For 0.12:

```
df = DataFrame(columns=['A','B'], index=date_range('1/1/2000', periods=20, freq='D'))
d_start = df.index[10]
d_end = df.index[11]
%timeit df.loc[d_start:d_end, 'A'] = 1
10000 loops, best of 3: 87.8 µs per loop
```

However, for 0.13.1:

```
10000 loops, best of 3: 203 µs per loop
```

I know that I shouldn't be looping on df.loc. However I find it concerning that there is a performance degradation. Is this a known thing? Thank you for your help.
