PERF: fixed performance degration becasue doesn't slice values array in to_native_types of DateTimeBlock
- [x] closes #25708 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
Slicing in [line 2181](https://github.com/pandas-dev/pandas/blob/707c7201a744c48c835f719421e47480beee2184/pandas/core/internals/blocks.py#L2181) only slice of i8values that make a whole of values array is processed in _is_dates_only function. This change was from pull #24543
```python
        values = self.values
        i8values = self.values.view('i8')

        if slicer is not None:
            i8values = i8values[..., slicer]

        from pandas.io.formats.format import _get_format_datetime64_from_values
        fmt = _get_format_datetime64_from_values(values, date_format)
```