The performance improvements in InDesign are more subtle than those in Illustrator. It doesn't use GPU like Illustrator or Photoshop. You'll notice there is no Performance preference like Photoshop, for example.
It is focused on these areas:
• Zoom in and zoom out
• Scroll using the mouse wheel or using Page Up or Page Down keys
(Panning with the Hand tool is not included)
• Generating page thumbnails on the Pages panel
Look at the "Performance Improvements" video on the Welcome Screen (Help > Welcome if you closed it) in the New Features section.
I'm using an Apple Magic Mouse and the scrolling is rather bad; too much lag. I guess the solution is changing to a wheel mouse. :/
Yes, Steve is right. Both Illustrator & Photoshop utilize the capabilities of GPU for fast rendering & zooming-in/out, panning and scrolling operations. InDesign is not yet using GPU for these. Performance improvements (2x) in InDesign are visible on normal Retina & non-Retina displays. But not so much on 5K Retina iMac.
Hopefully there are plans to update InDesign to make it work well with an IMac with 5K retina display; especially since a lot of designers will be using these machines and for your software to run sluggishly on them is sort of mind-blowing! I am currently using a PC and a MAC; the PC for InDesign. It's very frustrating! :/