Bees will build in unison when they can work jointly from both sides of the frame at the same time. With full sheets of foundation you'll find that the bees build out different areas on different sides of the same frame and may even spread out the number of frames they are working on due to the barrier that the foundation creates. Expecting bees to work uniformly on foundation is like expecting 2 carpenters to hit their nails on opposite sides of a wall at the same time with no communication. Foundation will also be built out rather wavy until the final bit for the same reason.
Bees working in unison will draw out comb faster than bees working with walls (foundation) between them. This is one of the primary reason I went to starter strips over foundation. I've found a hive on starter strips, working in unison, could draw out a complete frame of comb faster than bees on foundation could.