PERF: vbench for time-series index assignment in frame (GH5320)

PERF: direct index assignment in a frame was doing lots of work

closes #5320

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
frame_assign_timeseries_index                |   0.7157 | 398.8634 |   0.0018 |
```