Question PC lagging while connected to 4K TV, even if turned off

[sylleryum]

Hi All,
In my pc I have a monitor FHD (1920 x 1080) and a TV that has native resolution of 1366 x 768, they are connected in duplicate mode using 1920 x 1080 resolution and I can run games on 60fps just fine.
Now I switched my TV for a LG 43' 4K (ur7800) and regardless of the configuration or resolution, as long as the TV's display isn't checked as disconnected in Windows, the PC is lagging a lot for anything (not just screen, but input lag also), even if TV is turned off
for reference, here is sample of what it looks like normally while TV is disconnected:

https://imgur.com/1q7GFMG

View: https://imgur.com/1q7GFMG

and here is while TV is connected:

https://imgur.com/HVR2Whw

View: https://imgur.com/HVR2Whw

Drivers were reinstalled, TV configurations were reset but issue persists.
PC Specs: Ryzen 5600, RX 580, 16gb ddr4

 

[Aeacus]

1080p - 1920x1080= 2 073 600 pixels
4K - 3840x2160= 8 294 400 pixels

Your GPU, RX 580 is nowhere near powerful enough for you to be gaming on 4K. Hence the lag.

 

[sylleryum]

1080p - 1920x1080= 2 073 600 pixels
4K - 3840x2160= 8 294 400 pixels

Your GPU, RX 580 is nowhere near powerful enough for you to be gaming on 4K. Hence the lag.

Although the TV is 4k I'm using 1920x1080 resolution which is the same I had prior to the TV

 

[Aeacus]

Although the TV is 4k I'm using 1920x1080 resolution

Native 1080p or downscaled 1080p?

I think it is downscaled, rather than native. In downscaled, several close-by pixels act as one pixel (about 4-to-1 ratio, where to display 1080p on 4K, TV uses 4 pixels in a square to give out 1080p look), while all pixels in 4K resolution are still controlled.

To get true 1080p on 4K monitor, you need to use forced aspect ratio, whereby image is only 1/4 (quarter) of the screen on TV and not full screen.

 

[sylleryum]

If anyone comes across the same issue, I had to do 2 things to fix it:
1- for some reason Windows was switching automatically the refresh rate to 30fps, this can be changed in change the resolution of the display>advanced display settings
2-the input lag was solved by changing the TV's image mode to optimized for gaming