PERF: tab completion with a large index
from https://github.com/pandas-dev/pandas/pull/16326#issuecomment-348473324

If you have a very large index, ``_dir_additions`` (for tab completion) actually takes quite a bit of time

So what I would do is if the index is say < 100, use the currently _dir_addition, otherwise return an empty list! (its essentially too big to use tab completion for anyhow). can you make this change and add an asv for this (could be a separate PR as well)