PERF: perf improvments in indexing with object dtypes (GH5968)

[jreback]

closes #5968

-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
frame_iloc_big                               |   0.2836 |   2.8454 |   0.0997 |

[ghost]

That's a real boon for someone out there.

[ghost]

Hey now, it might be christmas again. frame_reindex is everywhere.

frame_dtypes                                 |   0.3270 |  66.4640 |   0.0049 |
frame_repr_wide                              |  22.0180 | 693.6400 |   0.0317 |
frame_iloc_big                               |   0.3091 |   3.5690 |   0.0866 |
frame_reindex_axis1                          | 327.7626 | 488.3307 |   0.6712 |
frame_reindex_both_axes                      |  47.7920 |  65.5576 |   0.7290 |
frame_reindex_upcast                         |  12.5876 |  16.9796 |   0.7413 |
packers_write_pickle                         |   1.0316 |   1.3046 |   0.7908 |
frame_reindex_both_axes_ix                   |  50.0793 |  60.2030 |   0.8318 |

[jtratner]

Nice work! What did you use to profile this and figure out where the issue was occurring?

[jreback]

oh started with @y-p putting up the issue about repr of a large frame being slow

first prob was the dtypes taking a non constant time ; 2ns was iloc

I stepped thru it to figure out