Latest Update [30 Nov 2016]
The latest MacOS Sierra 10.12.2 Beta seems to solve the mission control lag in my iMac 5k under default resolution. Under 3200×1800 there is still a minor lag or hesitation but I can live with that!
A lot of owners are reporting that there is a severe lag when engaging Mission Control. Or when switching desktop.
This is so especially when you have a few applications opened. The lag can be anything from a slight hesitation to a second “slow-mo” lag. Many felt that Apple should not release the retina iMac when such problem exists.
The retina iMac 5k is Apple’s flagship desktop computer and such problems really put a tarnish to Apple’s reputation.
Initial thoughts were of hardware issues. The retina iMac is uses one the best (mobile) graphics card but many felt that it is underpowered considering the it needs to power 14 million pixels.Apple should have used desktop series graphics card.
Apple released a few patches which tried to address the Mission Control lag issues. Some has reported a reduction in the lag issue but many including me still face the lag issue.
It is not something that will hamper your photoshop’s performance or your work performance but it is an annoyance because after all beside the Mac Pro this would be the next most expensive computer that Apple sells. And this group of customers paid good money and mostly professionals.
But fear no, there is a “not so pretty” work around to address this issue. If you go to System Preference and under Accessibility under Display, there is a function called “Increase Contrast”. Simply check that option!
Voila! The lag goes away! But you get ugly borders in your windows and the whole UI looks like crap.
So it is not a hardware issue after all! Which is a good news because it means it can be addressed by Apple after all!
This trick also works on any retina Macbook.
Update 12 July 2015:
With the new OS X 10.11 El Capitan Public Beta, the lag is fixed!!! Yippee! I think it must be the Metal Engine that did it. Mission Control now is smooth and sleek. Switching desktop is also split second. What a joy to use the retina iMac 5K now.