pandas sort_values significantly slower on Python 3.5.2 vs. Python 2.7.12
#### Code Sample, a copy-pastable example if possible

import pandas as pd
import numpy as np
from time import time
import sys

df_data = pd.DataFrame(np.random.randint(0,int(1e6),int(20e6)), columns=['pop_id'])
df_data['PL_dB'] = 50 + np.random.random(df_data.shape[0]) \* 100
df_data['Rx_dBm'] = 23 - df_data.PL_dB
df_data['noise_mW'] = (10.**(df_data.Rx_dBm / 10.)).astype('float32')

start = time()
df_data.sort_values(by=['pop_id', 'Rx_dBm'], ascending=[True, False], inplace=True)
df_data.reset_index(drop=True, inplace=True)

print("Sort took {:0.2f} seconds".format(time() - start))
print('Python version ' + sys.version)
print('pandas version ' + pd.**version**)
#### output of `pd.show_versions()`
## For Python 2.7
## INSTALLED VERSIONS

commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 25.1.6
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.0
statsmodels: 0.6.1
xarray: 0.8.2
IPython: 5.1.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.2
lxml: 3.6.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
## For Python 3.5
## INSTALLED VERSIONS

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 25.1.6
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.0
statsmodels: None
xarray: 0.8.2
IPython: 5.1.0
sphinx: 1.4.1
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.1
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: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
## Results with Python 2.7

Sort took 40.91 seconds
Python version 2.7.12 |Anaconda custom (64-bit)| (default, Jun 29 2016, 11:07:13) [MSC v.1500 64 bit (AMD64)]
pandas version 0.18.1
## Results with Python 3.5

Sort took 81.30 seconds
Python version 3.5.2 |Continuum Analytics, Inc.| (default, Jul  5 2016, 11:41:13) [MSC v.1900 64 bit (AMD64)]
pandas version 0.18.1
