When you are shopping for an LCD screen, the first thing the manufacturer will tell you, after the diagonal size, is the response time in milliseconds. Now you probably think that if the pixels take 25ms to change state, then that means your screen can update at 40 frames a second, which is just about okay for games.
However, this is because you are a Naïve Fool who has swallowed The Marketing Lie. That 25ms is the time it takes for pixels to change from black to white. If the pixels are changed to an intermediate grey, the response time is much, much longer because the liquid crystal molecules take a while to settle down at their new arbitrary rotation angle.
If you are scrolling black on white text on your LCD monitor (as I am now), the response time may be acceptably fast. But if you are playing Thief, where everything is a shade of dark grey, you’ll see response times as high as 90ms on some panels. That’s just 11fps. Or it would be if it was constant across the whole screen.