UPDATE:
Installed 2x32GB 3600 CL18 sticks, now my scrolling usages are as follows:
Adobe PDF: CPU=8%, GPU=~30%
Foxit PDF: CPU=18%, GPU: 10%
Chrome PDF: CPU: 22%, GPU:10%
Level of choppiness: Adobe<Chrome<Foxit
Moreover, iTunes movie playback usage on 1 4k screen is: CPU=25-30%, GPU=35-40%...