vaz I read on the Intel forums that the Windows drivers will enable dithering on 8bit and above displays. Your old display is probably a 6bit and maybe Ubuntu recognises this and disables dithering.
I have used Windows 10 in the last year on a 2012 laptop and it was perfect. When checking the driver in use I think it was using the same version supplied by OEM (2012 version) and not using a 2015> driver. So maybe it's just drivers, maybe it's Windows 10, without seperating it all (video output vs driver vs OS) - it's hard to determine where the 'point of failure' is.
How are you with games consoles/smart tv's? This isn't just an issue with PC's for me and I can't use Nintendo Switch for more than minutes.
Also iPhone 4 was fine for me, that had an LED display and I'm using a 2019 LED monitor now, so at least for me it's not LED.
Also it isn't necessarily eye strain thats the worst symptom for me (my eyes don't physically strain) it's neurological. I feel spaced out/over-focused on the screen.