A monitor's response times should be considered on a case-by-case basis. There's no universal system of measurement for response times.
It's not like a hard drive or something, where you can say this one is 250GB and this one is 500GB.
A lot of times, 8ms and 12ms panels are just older generations where manufacturers weren't as aggressive with their lies yet. These response times are usually more accurate. For example, 8ms may be the average response time for all color shifts for a certain Samsung panel, so they advertise that. Next generation, though, some yahoo over there says "hey guys look, it only takes 2ms to shift from this shade of grey to this next, slightly brighter shade of grey, let's advertise that and try to pass it off as an accurate description of our panel's speed even though it's not lololol."
