CRT pixels light up fast but they take much longer to "un-light" due to phosphor persistence.
P22R (red) phosphor decays at a rate of 0.7 millisecond per 10% decay (I looked it up). P22G (yellow-green) and P22B (blue) decay much faster at about 0.06 millisecond per 10% decay.
According to my crude calculations, at those rates it takes a red pixel about 5 milliseconds to go from full bright to half intensity. 10 milliseconds to reach 1/4 intensity. Call it 15 milliseconds to go from full bright to black. That's 15,000,000 nanoseconds.
<b>56K, slow and steady does not win the race on internet!</b>