you said if you open tonne of apps you can still notice the animation stall? have you updated it to Mojave? Is it better or the same?
Nothing like it was under OS X El Capitan and macOS Sierra. If I have 15+ app windows open, occasionally Mission Control animations can stutter for a split second when the animation sequence begins. I'm not trying to point out flaws to seed fear and uncertainty into your buying decision, but you asked the question so this is the truth. But you should know Mission Control animates app windows
live. We're not talking a static preview of the app window as it was before Mission Control started animating. It will show you any changes that happen to any app window before, during and after the animation sequence and also while Mission Control is open. As a software engineer myself, I would argue animating 14.7 million pixels at once – knowing fair well the app windows might change during and after the animation sequence – and to do that all perfectly 9 times out of 10 – is a pretty remarkable engineering achievement. These iMacs always run at 5120 x 2880, no matter what scaled resolution you choose. The only time it
doesn't run at 5K is when you select a low resolution mode, which isn't possible unless you enable the hidden setting in Display preferences.
No, I have not yet upgraded to macOS Mojave. But again, the Window Server now uses Apple's Metal 2 graphics stack and is the single reason system animations are so smooth now. It's like night and day compared to OS X El Capitan and macOS Sierra.