Assignment using .loc takes longer and longer
#### Code Sample, a copy-pastable example if possible

```python
import pandas, numpy
import time
df1 = pandas.DataFrame(numpy.random.random((4000,100)))
df = df1.copy()
x = 'ab' #Note: Issue does not occur if x = 0.2
for c in df.columns.values:
    df[c] = x #Note: This line is require to produce the issue.
    start = time.time()
    df.loc[1,1]=0.1
    end = time.time()
    if (c+1) % 10 == 0:
        print(end-start)

```
#### Problem description

I have copied another dataframe and am iterating over the columns. For each column I set the entire column to a default string (x). However, a second assignment using df.loc takes longer and longer at each iteration. The output I get from the above code is:

```python
0.004847049713134766
0.012796163558959961
0.016080856323242188
0.021880626678466797
0.026784181594848633
0.03387594223022461
0.04109668731689453
0.04407644271850586
0.049736976623535156
0.05252695083618164

```

As can be seen from the output, the second assignment requires ~5ms more on each iteration. This behavior leads to long wait times when using large dataframes.

#### Expected Output

I expected each assignment to require the same amount of time. For example setting x=0.2 in the above code gives:

```python
0.0003058910369873047
0.0003154277801513672
0.00029277801513671875
0.0002968311309814453
0.0003590583801269531
0.00033211708068847656
0.00032329559326171875
0.0003428459167480469
0.00043463706970214844
0.0002982616424560547

```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.8.0-44-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 20.1.1
Cython: None
numpy: 1.12.1
scipy: 0.16.1
statsmodels: None
xarray: None
IPython: 4.0.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None

</details>
