Lots of threads on this. Have you looked at them?
Yep. No answers that worked for me. All were unresolved or what was suggested didn't work.
It's Friday evening now, so I won't be doing this right away, but here's the test that I'll be running soon:
Load the same project on both Mac OS and Windows on the same computer (Windows on Bootcamp in the latter case). See if I notice a difference. Investigate from there.
My hypothesis is that it has to do with the OpenGL drawing being different between the operating systems.
More later, when it's not Friday evening and my wife isn't waiting for me...
Just an update on this...
I've tried the same project (and different projects) on my aging 2008 Mac Pro with a Quadro 4000 card and I get smooth scrolling up and down with all layers expanded.
What it seems like to me is a Mac vs PC thing. The iMacs we have at work don't exhibit the lag either. I also tried it on another PC (I only have access to 2 Windows PCs) and I get the same lag effect when scrolling vertically. Horizontally is fine, so I'm assuming it has something to do with the interface/switches.
It's one of those things that you don't realize until you get on a system that doesn't have it, and then you realize how much it affects you as you work.
I still intend to test this, but I'm traveling right now.
Great. Would love to see your findings on this. It seems to me that overall, AE runs better on a Mac than it does on a Windows PC. I have an iMac at work and a Windows machine at home with similar specs, except the Windows box has a better graphics card (GTX 780) and AE on the Mac seems to be faster overall, from the UI to even rendering.