An ideal panel has an absolutely linear time response gradient. This means the shift from red to grey takes just as long as the shift from white to black. Unfortunately, there can be no linear gradient of response time found among current LCDs (yet). But the various types of panels drastically vary in this regard.
MVA/ PVA panels, for example, show the worst gradient. While their minimum response time in parts is indeed less than 20 ms, the maximum response times is - at almost a third of the reproducible color space - up to four times higher than their minimum response time!
Opposed to this, IPS panels have a relatively linear response time gradient. Their maximum response time is at worst only 2.5 times over their minimum response time and in fact usually within a very small area of the lower color space (dark grey and brown).
For instance, if you are watching a movie on DVD, then its typically displayed colors are just within the color range that is known to be problematic for LCD to reproduce at average contrast and brightness values. An IPS panel with 30 ms displays this typical color space smoother than a MV/ PVA panel although the panel actually has the lower minimum response time (on paper). If it was a DVD movie in black and white at extreme contrast values (only black and white - no grey), then the MVA/ PVA panel would be superior since it can flex muscles with its lower minimum response time.
The response time, that is so often promoted, is basically worth nothing in practice. But manufacturers donít want or simply can't specify the response time gradient. The important thing for making judgments about ghosting is the inertia at average and commonly used contrast values. There is no movie and no PC game that would consist of black and white only.