Dragging sortables with long lists is slow. There is a significant lag when the lists get to be > 50 items. I found one very quick fix, which at least addresses sorting near the top half of the list.

In drag, simply breaking the loop once an overlapping element is found speeds things up dramatically. I added breaks as shown...

				if(overlap >= 0 && overlap <= 0.5) { //Overlapping at top
					ci.prev().length ? ma(this.element) : mb(this.element);
                    break;
				}

				if(overlap < 0 && overlap > -0.5) { //Overlapping at bottom
					ci.next()[0] == this.element ? mb(this.element) : ma(this.element);
                    break;
				}

Another possibility, which I haven't tried yet, is to track element positions during drag so we check for overlaps with the closest elements first rather than restarting at the top of the list each time.