2014 Macbook Pro 15" doesn't work well with 4k @ 60hz, obvious lag due to high WindowServer CPU utilization

Just got a Dell P2715Q, which supports 60hz @4k resolution. The monitor is gorgeous. There is little difference between the image quality as I can tell from MBP's retina screen. Very satisfied with the monitor.

But seems my new MBP 15" has some trouble dealing with it. It's the one without dedicated graphic card, and initially I found Iris Pro handled external 4k monitor at 60hz very well. However I soon realize there are severely lag happening. Finally I pin it down to WindowServer. The window movement as well as window creation/change is very slow, the most obvious example is using IME, which would create a dynamic new window. With new 4k monitor connected, at first it's still very fast, but later after several minutes, it would become slower and slower, then you would find the IME cannot catch up with input speed.

Another example is moving window around, you can see the lag happening as well. Or start mission control, in which case you would see it seems running on 386.

I've searched a lot and finally disable several options mentioned on http://www.macissues.com/2014/10/20/prevent-the-windowserver-process-from-draggi ng-yosemite-down/ as well as "Displays have separate spaces" in the Mission Control. The last one give me best shot, but I lost an important function! But since it's performance downgrade during the time, I would consider it's a bug of Yosemite windowserver.

Currently I still experience lag from time to time, especially when using IME. I am very disappointed, don't know how long it would take Apple to fix the issue. It's the latest MBP 15", I really think I should expect more than this.

Anyone has other ideas to solve this? The MBP is new, I don't think I would reinstall the OSX...