When transitioning from one frame to the next, the M&M’s sign and all components of the ABC sign update all pixels on the sign at once, demonstrated here in less than one of my video capture frames, so we know it’s less than 1ms. The different components of the ABC sign seem to update at different times because of a sync issue, not an LED technology issue.
LCD displays will lock to incoming signals with a variety of timings, often anywhere from ~57Hz to ~63Hz, depending on the display. When you tell a graphics card to output at 60Hz, it is doubtful that it’s putting out a true 60.000Hz. Depending on the make of the card, the drivers installed and the phase of the moon, the actual refresh rate will vary quite a bit. LED signs tend to maintain their own clock, regardless of the video signal that might be driving it. It is likely that without a genlock signal keeping the system in sync, the source signal will not provide frames at exactly the same frame rate that the LED sign is displaying them. The more disparate the source and display refresh rates, the more dropped or doubled frames you will observe, which will be visible to the layman as stuttering.
The Reuters sign, however, DOES update one row at a time, just like an LCD. The passing of the raster takes about 14ms, as we might expect for a sign running at 60Hz:
I cannot speculate as to the implications for their sync mechanism, except to observe that the rasters of the three screen segments visible in this video seem to be in good sync, though the frames themselves do not.