pydata/pandas — pull request #5462 by jreback (2013-11-07 13:06:16+00:00)
Title: PERF: performance improvements on isnull/notnull for large pandas objects

1000 x 1000 frame
```
-------------------------------------------------------------------------------
Test name | head[ms] | base[ms] | ratio |
-------------------------------------------------------------------------------
frame_isnull | 3.1417 | 121.2864 | 0.0259 |
```

now evaluates block-by-block rather than column-by-column