Hmm how to put it in an understandable way.....
The reason why you can't drop humongous bitmaps in Construct without a huge penalty is the same reason why you can put a jillion transparent moving objects rotating and zooming at a high screen resolution and still get 60fps. Let's see RPG maker do THAT! (and oh, hi shaders!)
It's called hardware acceleration, and every image you use has to be first loaded into RAM and then transferred to the video card memory, which is limited (currently between 128 and 1024Mb TOTAL).
you will ALWAYS have to work around limitations, with ANY computer system. It's a fact of game development. Please, try to learn the tricks that are selflessly being offered.
(Btw that screenshot: half-size the background, then stretch it. Use one layout per room, load textures on layout startup. TADA!)