You will notice this as soon as you use a low dpi device again for any length of time. First 10 minutes you see the pixels. After that you stop seeing the pixels.

Viewing distance plays a factor too. I realized 4k tvs are useless to me because I can't perceive 1080p pixels unless I'm comparison shopping in a TV show room from 2 feet away (probably why they don't let you experience appropriate TV viewing distances in these spaces in the first place).