Have done a clean Sierra install, and still was getting stuttery performace with Designer. After hiding rulers, it seems that the stuttering has been taken care of - so something with Rulers being visible is slowing everything down. Is there something I can show with Activity Monitor that will be helpful for you?
I wanted to chip in and say since 1.5 performance has been really sluggish for me as well, both on my laptop screen and external ACD. It's really rather annoying. Dragging things around is sluggish and stuttery, and resizing has a 1-2 second delay before it catches up. I'm on a 2012 rMBP, latest Sierra update.