The reason Monday Boarding Groups go more quickly is that there are fewer BGs available, due to the 6 pm drop for deluxe resort guests (evening hours perk.)I am not sure if it is still the case, but for awhile the Monday Individual Lightning lanes for Cosmic Rewind went significantly faster on Mondays. This was the main reason we avoided Mondays at EPCOT. I know it is not as bad as it used to be, but last I had looked you could still see a noticeable difference in how fast the Cosmic Rewing LLs went away.
It doesn't mean the crowds are heavier on Mondays; it can be just the opposite. The evening hours guests usually don't begin arriving until 8 pm.