Take the time to look closely at the original mosaics and see just how rough they actually were. Close to they sometimes look like they're wandering all over but from a distance they look fine. This, in my view, does show that they worked at a set rate, not fast as such, but they didn't worry overly much about making them too neat.
Notice on the image below, a detail from a double guilloche border pattern, how they have been left with too much width on the strands so they've just added another line of the red tesserae.
They have kept to The Rules in the way the red tesserae have expanded into two lines, the use of keystone shapes on the curves plus the exception to that Rule where triangle shapes are used for the black/dark green lines.
Image copyright Helen Miles