They do darken that fast (not fast enough you can't catch it in a high speed camera, but much faster than a frame). Most of the apparent persistence in the CRT

They do darken that fast (not fast enough you can't catch it in a high speed camera, but much faster than a frame). Most of the apparent persistence in the CRT comes from the retina/camera exposure, not the phosphor. A CRT has a sharp peak of light that quickly falls off, but the peak is bright enough that even though it is brief, when averaged out in an exposure in the camera it still appears bright enough to form an image.