You might be thinking of the issue early adopters had with TVs that only did 4K/30 over HDMI. At 60Hz, it's like any other computer monitor.

I detect no lag when moving windows around. It might not (or might -- I simply don't know) be good enough for gamers, but it's definitely smooth enough for development, document, or web browsing (aka productivity) tasks.

144Hz monitors do produce a very noticeable difference for gaming if you can rival the refresh rate with in-game FPS, outside of gaming I've only found it noticeable when moving windows about.

For productivity I'd go for the 40" 2160p over a lower resolution higher refresh panel.