It would need to redraw the whole screen to account for the phosphor decay. To do that with line resolution and an NTSC signal, you’d have to redraw it roughly 1500 times per second (60 fields of about 250 lines). You’d draw the current line at full brightness and “decay” the rest of the frame according to that phosphor persistence. Since there is some quantization, you could reduce the frequency of decays as the line gets older.