Is a unlimited DW board faster to paddle in flatwater than a 14ft flatwater board?
Depends on what unlimited boards but in most cases yes, our 17'11 dugout ocean board is pretty close in speed to our 14 x 21 edge pro flatwater board. Maybe the glide speed of the edge pro will win out over a distance but the unlimited is surprising fast in the flat.
Is a unlimited DW board faster to paddle in flatwater than a 14ft flatwater board?
I would say that most DW specific UL boards are slower on flat water than a dedicated flat water board in a 14' length. There may be the odd exception like what Jacko said.