Ram preview plays back slow on 4k monitor unless I hide timeline panel
When your timeline panel is displayed on a large monitor, ram previewing is seriously compromised. If I simply hide the timeline, ram previewing works perfectly. I'm guessing that the code behind the timeline is super old, and needs to be free programs for bigger monitors.
The latest (November 2019) release of After Effects includes major improvements to our preview architecture. You should see reliable, real-time playback of cached frames.
Stéphane Augé commented
Same issue here. It is a bug related with 4k monitors. It appeared with CC 2015, CC 2014 is working perfectly smooth with 4k monitors.