PERF: groupby regression
https://github.com/pandas-dev/pandas/pull/16413 may have slowed down `time_groupby_series_simple_cython` (while speeding up other groupby ops)

```
$ asv continuous d5a681bfa~1 d5a681bfa -b groupby.groupby_multi.time_groupby_series_simple_cython 
```

```
· Creating environments
· Discovering benchmarks
·· Uninstalling from conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
·· Installing into conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt..
· Running 2 total benchmarks (2 commits * 1 environments * 1 benchmarks)
[  0.00%] · For pandas commit hash d5a681bf:
[  0.00%] ·· Building for conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt..........................................................................................
[  0.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 50.00%] ··· Running groupby.groupby_multi.time_groupby_series_simple_cython                                                                                                                                                                                                                                    5.04±0.09ms
[ 50.00%] · For pandas commit hash 5fe042f5:
[ 50.00%] ·· Building for conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt..........................................................................................
[ 50.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[100.00%] ··· Running groupby.groupby_multi.time_groupby_series_simple_cython                                                                                                                                                                                                                                    3.79±0.09ms       
       before           after         ratio
     [5fe042f5]       [d5a681bf]
+     3.79±0.09ms      5.04±0.09ms     1.33  groupby.groupby_multi.time_groupby_series_simple_cython

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```