I've never opened a 20,000 page document, but I've opened documents of several hundred pages with no noticeable issues. Pressing either Space or PgDown instantly moves the page down, no scrolling animation. The option still does not exist, but it seems that the feature doesn't exist either.
IIRC the eventual fix was to have it respect the global compositor settings, so if you have animation speed set to instant, while it does actually smooth scroll, it does so instantly. However, when it was introduced, it did not; for similar reasons (i.e. I don't like nausea), I had animations disabled, but Okular did its thing, and slowly.
It took some time for it to land though and not without the kind of discussions that you see on the Gnome bug tracker. It was pretty disheartening.
