There's a massive difference between a CRT at 30 Hz and an LCD at 30 Hz. A 30 Hz CRT is terrible in direct proportion to how bright it is, and it very visibly flickers. 60 Hz CRTs also visibly and annoyingly flicker with eye saccades, or if you wave your fingers in front of the screen. CRTs need to approach 75-85 Hz before they are all-day tolerable with white backgrounds.

LCDs have a continuous backlight and the liquid crystals need to change orientation to change frame. There's no flicker as long as they're showing the same frame. Here the flicker is noticeable as jerky motion, and is mostly just a problem for video, games, mouse cursor motion, smooth scrolling, etc.