bug#23377: 25.0.93; Completion is extremely slow for insert-char

[Drew Adams]

> Seems like a better way forward is to add a config var, which defaults
> to disabled.

+1 for the option.

But I think it should be enabled, not disabled, by default.

The typical, and generally better (IMO) reflex is to use a
good pattern to match, not just shoot blindly: C-x 8 RET TAB.
Exceptional users can easily disable it.  Most will take
advantage of it.  In practice, it shouldn't take much typing
to radically improve the performance.

IOW, if the main reason to disable it is for performance in
an extreme use case (C-x 8 RET TAB), then that should not
guide the default behavior.

(Of course, the choice of default behavior is much less
important than is the choice of letting users decide, by
giving them an option.)