PERF: use fastpath=True in Index methods (delete/drop/insert/etc)?
Been hit by this when optimizing index-oblivious Blocks. In my case,

```
is_deleted = np.zeros(len(index), dtype=np.bool_)
is_deleted[deleted_loc] = True
index = index[~is_deleted]
```

was still a lot faster then than `index.delete(deleted_loc)`.

It appears, that delete doesn't add `fastpath=True` to ctor and that triggers type inference for string (object) indices. There seems to be plenty of methods that do it the same way and hence are slow too, can we do something about that?
