P: Editing on a 4K screen is VERY slow

Hi There,

I just got out of a chat with support and asked me to do a feature request here.
I told her it's a bug!

Here we go.
I do architecture and skyline photography.
To be able to do what I want I am stacking a lot of layers and use parts of each picture to create the final image. I am on a very decent Mac Pro 2013 with 64GB of ram and D700 graphics cards. 

The problem I have lies in the redraw times when enabling or disabling a layer. 
If it works fast it takes 3 or 4 seconds to update the picture on my screen. (so really slow)
However when it really slows down it can take up to a whopping 30 seconds before the picture gets updated. Zooming in can take this long as well.

So I have been investigating a lot I even brought my mac in for repair but everything was OK.
It should be because 4K movies play without a hiccup.
Anyway, a friend of mine with the dame configuration sold his mac after 2 months of trying. That's how frustrated he was. He lost 3000 dollars on the sale. (his choice)

Today I went to a store for graphics professionals to check out the new Imac 5K with 4GB Vram and 4Ghz i7 processor. it had the exact same problem!? Also with an external 4K screen attached.
He told me that I should start thinking about porting to windows. I could check out their main video editing system which would eat my mac pro for lunch. To our surprise, same problem!! clicking on layers or updating/changing adjustment layers takes forever.

So our conclusion was that Photoshop is the problem! can't believe it, but it's true!
Some questions you are going to ask me I will answer now:
I tried internal and external drives
I tried ALL different scaling modes available. (native 4K is fastest but still slow)
The old photoshop (CC15/15.5) had the same problem.
I also tried factory ram instead of OWC.
I tried many different files.
We tried different 4K screens LG, Eizo, Dell, Samsung and the 5K screen on the imac itself.
I also tried my macbook pro on 2 of those screens and that's even slower.
We also tried a macbook for fun,.. even worse.

Some pointers.
-Zooming way out makes it way faster (33.33% for instance) make it bigger and it get's slower.
Fullscreen or zoomed in is unworkable.
-On a HD or 1920x1200 screen there is no problem at all on the same machines. There it's instant.
-When Lightroom is open in the background the problem gets worse.
-I also have the feeling that smartobjects with curves in the same document makes it worse (but that's more of a hunch)

Besides all this I am quite sure you guys are aware... you must be!
Anybody doing composites will probably use even more layers and/or bigger files.

I hope you can send me a beta where this is issue is being addressed. I wouldn't mind giving feedback on it either.

Please, please, please Help!!!
Best,
Mark den Hartog