JuDDering is definitely there when I play a game capped at 30fps or watch a movie at 30fps on my 60hz displays. Only 60 or above is silky smooth.
I don't think you mean juddering. Jerky gameplay, sure, but that isn't judder. Judder is a very specific thing caused from frame duplication or frame skipping. You're talking about in-engine stuttering and jerkiness, which would be obvious even on a 30hz display.