Have you ever seen high speed footage of a CRT in operation? The phosphors on most late-80s/90s TVs and color graphic computer displays decayed instantaneously. A pixel illuminated at the beginning of a scanline would be gone well before the beam reached the end of the scanline. You see a rectangular image, rather than a scanning dot, entirely due to persistence of vision.

Slow-decay phosphors were much more common on old "green/amber screen" terminals and monochrome computer displays like those built into the Commodore PET and certain makes of TRS-80. In fact there's a demo/cyberpunk short story that uses the decay of the PET display's phosphor to display images with shading the PET was nominally not capable of (due to being 1-bit monochrome character-cell pseudographics): https://m.youtube.com/watch?v=n87d7j0hfOE

Interesting. It's basically a compromise between flicker and motion blur, so I assumed they'd pick the phosphor decay time based on the refresh rate to get the best balance. So for example, if your display is 60 Hz, you'd want phosphors to glow for about 16 ms.

But looking at a table of phosphors ( https://en.wikipedia.org/wiki/Phosphor ), it looks like decay time and color are properties of individual phosphorescent materials, so if you want to build an RGB color CRT screen, that limits your choices a lot.

Also, TIL that one of the barriers to creating color TV was finding a red phosphor.