PERF: regression in getattr for IntervalIndex
Master:

```
In [14]: idx = pd.interval_range(0, 1000, 1000) 

In [15]: %timeit getattr(idx, '_ndarray_values', idx)
1.29 ms ± 30.6 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [16]: %timeit idx.closed
321 ns ± 2.66 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
```

while on 0.25.3:

```
In [13]: idx = pd.interval_range(0, 1000, 1000) 

In [14]: %timeit getattr(idx, '_ndarray_values', idx)  
90.5 ns ± 2.09 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [15]: %timeit idx.closed 
105 ns ± 1.61 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
```

(just checked a few attributes, didn't check if it is related to those specific ones or getattr in general)

I think this is a cause / one of the causes of several regressions that can currently be seen at https://pandas.pydata.org/speed/pandas/ (eg https://pandas.pydata.org/speed/pandas/#reshape.Cut.time_cut_timedelta?p-bins=1000&commits=6efc2379-b9de33e3)