For example in the first town with the deathclaw there are tons of buildings and at least 3-4 can be entered and used as scouting places upstairs. Does that require less graphical power than settlement buildings since there is a settlement size limit??

Not sure but at the stage I'm at with my settlement, I've glitched the size limit beyond belief, it's only just starting to cause slowdown.

Just asking because after building a junk fence around the settlement and then 3 floors 2x5 almost without any walls, the size limit was reached. It was the pit stop place. Doesn't seem to work.

It's a complex matter, but yes, pre-built objects need less resources than player placed objects. They can be heavily optimized for lighting, rendering and collision Detection, which is difficult for dynamically placed objects.