completion delays while typing, even when completion.show == "auto"

[NoSuck]

Version info (see :version):
qutebrowser v1.6.2
Git commit:
Backend: QtWebEngine (Chromium 69.0.3497.128)

Does the bug happen if you start with --temp-basedir?:
No, of course not.

Description
I noticed that response while typing in the command bar is slow, even when completion.show is set to auto. It's as if history.sqlite is being queried ala always, just with hidden results. Perhaps @rcorre can confirm this?

I would expect the query to be executed when a completion is actually requested, e.g. completion-item-focus is called.

How to reproduce

• Have a large ratio of history.sqlite size to computing capability.
• Set completion.show to auto.
• Enter the command bar with :open and start typing.

[rcorre]

From a cursory glance at the code, I'd say you're right. completion.show only affects whether show is called on the widget. set_pattern is called whenever the text is changed, regardless of completion.show.