Since no one post any video to illustrate how bad the UI is. I just made a short video to show the UI performance on my cMP. So now, we can make comment base on the same video, same standard.
Mac Pro 5,1, W3690, R9 380 4GB (as per my signature), the CPU was under stress (about 90% usage on video encoding) when I capturing this video. But that generally doesn't affect the UI performance. And a Mac Pro suppose to be busy all the time anyway. So, I capture the video now, rather than wait until it has idle time.
The focus of this video is on the UI smoothness. Therefore, I use my iPhone to capture the screen rather than using Quicktime to capture the screen (which may introduce UI lag, or unable to capture everything at 60FPS). And I only use 720P to make sure the capture is smooth, which also intentionally make the details hard to read. Please only focus on the UI smoothness.
This is what I generally have, and it looks OK to me. So, please feel free to comment if it's more of less the same on your cMP. If yes, which part (e.g. which second on the video) shows the "lag" etc. May be just I am too casual about the UI, and don't care it.
Also, if you want me to test any UI performance for comparison, please let me know. However, please tell me the exact procedure on how to achieve that. e.g. I can't quite remember what is Expose, I think we only have Mission Control now. So, please tell me the exact UI / function that you are looking for.