On Nereus and Orbit (before I moved Orbit to Unity) I would experience exactly the same issue where shrinking the game window would improve frame rate performance. I fixed it on Nereus by limiting the number of objects in my scene. I would randomly generate an asteroid field, and found that if I reduced the number of objects in the level to <1000 performance was rock solid, anything like 1500 to 2000 and it would really start to chug on my laptop BUT weirdly, shrinking the window would bring the frame rate back right up.
Another prototype I worked on had a massive tilemap, I'm talking a layout size of 8960, 5040 with a tilemap covering a lot of it made up of 16x16 tiles. Again, bad performance at full screen, solid 60 when reducing the window size, reducing the internal rendering resolution and all that didn't seem to make any difference.
On my desktop (which is very powerful) I get a solid 60 on the Airscape demo fullscreen. I can't test my integrated chip laptop right now will edit post with results later.