Yeah, a 4K display is about 8 megapixels so at 60fps you need to write 480M pixels per second. That's feasible with a single CPU depending on the complexity of the rendering. Multi-core can get more work done per pixel. You'd still be writing highly optimized code to render fairly simple things if they require a full screen update.
That's assuming you rewrite the whole screen every frame. Most productivity apps don't. Windows (and X11) had this whole infrastructure about managing dirty regions.
Computers with GPUs are now fast enough to blast through recomputing the entire screen's contents every frame, but perhaps for efficiency reasons we still shouldn't?
