My 5,1 is close to as good as it gets (12 x 3.46, 128gb, 1tb PCIE SSD, GTX1080) and is running the latest Nvidia drivers and the latest build of Sierra...and yet I have UI lag like you can't believe. Expose is sometimes useless, swiping between fullscreen windows can sometimes be painfully slow. Safari is almost unusable at times. Just added the GTX1080 today and it made a very small improvement, but not much better than the GTX680 before it. It didn't used to be like this, but my usage case hasn't changed much on this machine and the UI performance has continued to get worse. Anyone else experiencing this? Very strange and exceedingly frustrating for it to feel so slow when I know it's not...