why is my 4k video choppy

Hi, all. I just completed my first build from scratch and am running Deepin. I don't game, and I thus opted for an integrated graphics card (Intel i5 9600K) rather than having a separate GPU. I am disappointed in the 4k video quality. I have tried YouTube test videos as well as a downloaded a 30s test clip from Captain Phillips, which was also choppy, which makes me think my internet speed is not the primary culprit. I use a Dell U2718Q monitor that I used as a second monitor with my previous Apple laptop and did not notice significant 4k lag. I also know its specs are sufficient. The monitor is connected to my MSI MPG Z390 Edge AC motherboard via DisplayPort to DisplayPort cable. My OS resolution is set at 3840x2160. I thought that turning down the "Display Scaling" (from 2.0->1.0) helped at one point, but I was not so sure on subsequent viewings.

My searches thus far have indicated to me that the i5 9600k (and its integrated graphics) should be able to handle 4K video. Is there some setting I need to be tweaking with the Intel graphics chip? Deepin has something called the "Graphics Driver Manager" which purports to accelerate the Intel graphics, but I noticed no difference when using it. Thank you in advance.