I have a 4K TV hooked up as my monitor with an older video card. Unfortunately, this video card can only output 4K at 30Hz which isn't optimal, but for my purposes is just fine. I'm not playing games that require fine tuned actions.
I recently changed my display to 1080P for an online presentation and then back to 4K. By default, 4K was running at 60Hz, however, this made the text slightly blurry because HDMI was compressing the signal. Set it down to 30Hz to fix that, but then the text was jaggy and not smooth. Also, my mouse was really laggy as I moved it around the screen.
When I changed back to 4K 30Hz, Windows 10 and the TV negotiated using HDR (High Dynamic Range) for display. Normally when Windows 10 negotiates a setting, that's preferred, but not in this case. When I disabled HDR in display settings, my fonts were smoothed properly and the mouse lagging stopped.