Using ipython6 with Jupyter notebook, you see much longer delays for tab-completion on large variables (arrays ~1GB in size in memory) which don't occur with previus version.
In the meantime, looks like there is a way one can disable jedi in ipython with
Specifically you can use
ipython locate profile to find your current profile directory, and edit the
ipython_config.py to add
c.IPCompleter.use_jedi = False