Issue with UHD (4k) screan

Hello everyone, i am running on an HP omen with ultra HD screen (38402160). When i run the client, it runs for example on resolution 1280720. the screen area that the client cover is far greater than 1280720. it seems to at least cover 3/4 of my screan. which is basically around 30001800. Running the 1920*1080 on the client makes it larger than the full screen, and can't press the play bottun. the issue is that:

1280720 => 30001800 on my screen, and the client becomes real laggy like 10 FPS. i tried the following but none worked: Run as admin/ reinstall and full download/ run in compatibility mode/enable low spec mode.

any help? appreciated and thanks.