dramatic performance difference b/w DataFrame index.lexsort_depth and index.is_monotonic
```
len(alphas.index.levels)
```

> 2

```
%time alphas.index.lexsort_depth
```

> CPU times: user 0.03 s, sys: 0.00 s, total: 0.03 s
> Wall time: 0.02 s
> 1

```
%time alphas.index.is_monotonic
```

> CPU times: user 10.22 s, sys: 0.00 s, total: 10.22 s
> Wall time: 10.22 s
> False
