Nearly all monitors stated response time is in gtg so if 5ms is stated, then 5ms it is.
However, response time is not an exact science, it merely is the least amount of time that the pixels changes from one shade of color to another in a test environment. Only the fastest time is used. All other recorded response times are thrown out with the trash.
Monitor A may be rated at 5ms, but it's slowest record response time could be 200ms. Monitor B may be rated at 2ms, but it's slowest record response time could be 250ms. You can't know the "truth" about a monitor's performance unless you can get access to that type of test data.