LED Matrix Shift Delay
That is called POV - Persistence of Vision.  The retina of your eyeball retains a static image for a few milliseconds even if the image turned off in less than a microsecond.  This is what makes television and movie images appear to move smoothly.  The faster the image moves the more blurry it will appear.  This is a common problem with all "Times Square" displays.

Changing the amount of time the image is on helps reduce the blurring.  For example, in your code each image is illuminated for 48 ms, then changes almost instantly to the next position.  If you change it so each position is lit up for only a few milliseconds, then is dark for the rest of the 48 ms before it moves, this will reduce the apparent blurring; the image will have more of a stroboscopic look.  But a complete cure includes moving the letters so slowly that the blurring is minimal.