perf regression in vbench frame_drop_dup_na_inplace in v0.10.1 vs v0.10.0
![perf](https://f.cloud.github.com/assets/1820866/377694/d848aef4-a4df-11e2-8715-2801ceb3075b.png)

c749c18    5.885601
d5b7907    5.922031
ffe4030    5.872154
375248e    6.886816
4367336    6.928682
