I'm on Windows, using a UHD 4K monitor and running 175% display scaling.
After Effects scales the UI correctly, but the performance of the Timeline panel especially is atrocious. Merely trying to scrub the timeline is very laggy. There are also visual glitches, like vertical black lines, and the playhead not redrawing correctly when using J & K to move between keyframes (the playhead gets disconnected from the line in a redraw glitch). Even doing a RAM preview on a fully cached timeline causes frame drops.
If I use the Windows high DPI settings dialog to override the scaling behavior (set scaling to be performed by System), all these performance and glitch issues go away. Scrubbing the timeline and resizing panels is smooth, and no visual glitches appear. Unfortunately, the entire UI is drawn in low res - pixellated and blurry, so it is not usable like this.
This has been a long-standing bug and I reported it over a year ago, thinking it was a graphics card issue, but it is clearly related to severe performance issues when doing high DPI scaling.
Here is the original post, which has screenshots and more details about the problems that occur:
https://forums.adobe.com/thread/2312838