4k TV GUI lag

I’m using a 4k TV, HDMI2.0, 60hz, 3840x2160. Everything runs smoothly, no mouse lag, but when I click and drag MIDI notes in Cubase 8.5 they drag behind and it’s very choppy. I am using an AMD Radeon RX460, the cheapest card I could find with HDMI2.0, just to test this setup.

Is anyone running C8 at 4k/60hz smoothly?

What is most likely causing this?

I’m considering borrowing a high end video card to see if that’s the issue but I’m curious if it may be Cubase itself or something else. If anyone has any expertise in this I’d love to hear your thoughts! Let me know if I can provide any additional info.

Thanks
Henry

Windows 10
AMD 8core 4ghz
32GB RAM
Cubase 8.5
43" LG 43UH6100 4k