Yes, unfortunately in a very large score these adjustments can be slow, because each adjustment requires a complete calculation of the spacing of at least that system, and sometimes more.
We do certainly want to make it possible to adjust note spacing with the mouse, but we would very likely not be able to show the whole system updating in real time and still track the mouse position smoothly, which is why we have not done this as yet. We would probably end up having to “grey out” the music and show only the handles and column markers themselves during the edit, then “committing” the edit once you finish dragging.
To adjust the offset of the up-stem smaller notehead, first select it in Engrave mode, then set the Voice column index property to 1, which will cause it to jump out to the right. Then use the Voice column spacing offset property (not the Voice column X offset property) to adjust that column leftwards by setting a negative value (around -1 space should suffice).