If you're talking about supersampling, that is where you run the game at 1440p or 4k on a 1080p monitor, and it 'downscales' the image to fit. This means you get a sharper image but it takesmore GPU power to run, since you're basically playing the game at 1440p or 4k instead of 1080p.
If you're talking about changing your monitor resolution to something lower, like having a 4k monitor but setting your PC to output 1080p (and the monitor upscales it to fit 4k) then essentially you're playing the game at 1080p so your performance is much higher but the details are all worse.