One 43" 4K monitor (or TV) has been my go-to for the last 4 years. Plenty of screen real estate without the physical limitations of dual monitors in different orientations. And at 43/42 inches, PPI is just right for me - no scaling necessary.
It hasn't been an issue in my experience. rtings.com rates input lag on tvs (as well as how well they work as monitors).
Most modern units have game mode and/or PC mode which I assume disables some processing to drive down lag. On my LG C2 OLED, changing the input type to "PC" results in a visible flicker (just one flicker) when the display is shifting into some new mode.
