PERF: fix perf issue in tz conversions w/o affecting DST transitions


```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
datetimeindex_normalize                      |   3.3297 |  83.0923 |   0.0401 |

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [fc88541] : PERF: allow slice indexers to be computed faster
Base   [160419e] : TST: fixes for 2.6 comparisons
```