Oh wow. That’s pretty crazy. But with only that one bit of info – I can think of a dozen different things that maybe it could be. I think it would be good to collect just a tiny bit more data about the problem if you can:
You are running at least half a dozen other apps. All are probably vying for limited video RAM – which is not overly abundant on most iMacs. I would definitely see if quitting all those apps and a reboot changes anything.
Is this always repeatable. Is it always laggy? Or just some pages? Or just some projects? Is it laggy when you add the very first stack to a blank page?
Are you running any utilities that might be doing some work in the background when you drag stuff: window managers, paste-managers, etc. If so, then I think it’s worth trying with everything turned off – or just quickly make a new user-account and see if the problem exists in the new-user.
Is it hardware dependent. If you have another machine or a friend that you can share the file with – does different hardware make the problem better or worse or no different?
I can say that I have never experienced this problem – by which I mean: I’ve seen lots of slow project files, it’s not hard to build a slow page – but I’ve not seen one that can be made faster simply by making the window smaller.
I just tested on a sort of medium-big Foundation project I have. It’s not as slow as your example, but adding new stacks does lag a bit. But changing the size of the display doesn’t seem to have any impact.
I have a display that will do the crazy resolution of 3840x2160 – even on a 28" display everything is unusably tiny. Still, I don’t see this issue. This likely means that this problem is something isolated to:
- your hardware
- your environment (utilities, other running apps, your network connect for downloading online warehoused images, etc.)
- your project file – perhaps some particular stack has found a way to be slow in this strange way.
fun screenshot of very large display stacks – scaled a lot for posting, obviously. Look at that teeny tiny little menu bar at the top. Who uses these modes? Why are they even here?