Can completions be cascaded somehow? The first tier would show all the
common first words, e.g. ... AEGEAN APL GREEK ... and then selecting
something from the first tier would cascade down to the second tier.
The slow display should only happen when the list is really long,
i.e. basically if you hit TAB with an empty minibuffer. So we could
indeed easily use a different completion behavior in this case.
When I type TAB in the empty minibuffer, I really want to see all
completions even when the list is really long, to be able to use isearch
to find a completion candidate etc. Everyone who wants to narrow the
completion list can type the first word like "latin TAB", so I think the
current completion behavior is satisfactory.