I've got the same in Safari on a 2015 rMBP, but Chrome handles it fine. Safari's always a bit odd with parallax effects, because of the way it handles scrolling—it'll keep moving the page downward even when it's not rendering, meaning whenever it doesn't necessarily recalculate the position every time the image moves. Makes for choppiness, but I'm not sure there's much to be done about it.
I've got the same in Safari on a 2015 rMBP, but Chrome handles it fine. Safari's always a bit odd with parallax effects, because of the way it handles scrolling—it'll keep moving the page downward even when it's not rendering, meaning whenever it doesn't necessarily recalculate the position every time the image moves. Makes for choppiness, but I'm not sure there's much to be done about it.