> Absolutely.  Like with any similar interaction in
> Emacs the delay before starting should be a user
> option.  Users are different, and user machines etc.
> are different.

You are right, with a small delay lazy-highlighting is responsive.

In the attached patch I also added a separate variable that you asked for
programmatic use, changed text to "currently visible on the screen" and
mentioned lazy-highlight-cleanup.

The patch is extensively tested for various scenarios: