Video lagging when playing 4k 60 fps video on fullscreen but on maximized video everything is fine on newest k lite codec version

Like in the title: Video is lagging/dropping in framerate when playing 4k 60 fps video on fullscreen but on maximized window, everything is fine on newest k lite codec version.
I reinstalled k lite codec pack program, ran all tweak tools, and my video card is laptop rtx 4060 so this isn't hardware issue. How to fix this issue so I could play 4k 60 fps video with .mkv extension in fullscreen not maximized windows so it won't lag?

I assume your laptop has two GPUs (also Intel/AMD integrated one)? I the Windows settings tool, go to Display settings, then advanced. There you can choose which GPU should be used by MPC-HC (and other apps). The integrated one is probably used by default.

If that does not help, then do this:
In LAV Video Decoder settings, D3D11 should be selected as hardware decoder, and you should change hardware device from automatic to your 4060.
Automatic (native) mode is normally more efficient, but someone else also reported framerate issues with it. I think it may be due to a NVIDIA driver bug.

Changing settings in LAV Video Decoder to D3D11 and rtx 4060 graphic card fixed the issue, thank you.

Wasn't it set to D3D11 already?

Test the other hardware decoder options. I am curious if problem only occurs with D3D11 automatic.

Which version of the NVIDIA driver do you have?
Can you test if it still happens with yesterdays new driver version?