performance regression in DataFrame.sum?
```
In [29]: df = DataFrame(randint(2, size=(8e6, 10)))

In [30]: time df.sum()
CPU times: user 1.21 s, sys: 0.03 s, total: 1.24 s
Wall time: 1.23 s
Out[30]:
0    3999325
1    3998264
2    4000047
3    3997902
4    4001078
5    4001965
6    4001701
7    4000482
8    3997581
9    4000691
dtype: int64

In [31]: time df.mean()
CPU times: user 0.38 s, sys: 0.00 s, total: 0.38 s
Wall time: 0.39 s
Out[31]:
0    0.5
1    0.5
2    0.5
3    0.5
4    0.5
5    0.5
6    0.5
7    0.5
8    0.5
9    0.5
dtype: float64
```

Should sum be 3.15x slower than mean?
