and what are those "same conditions"? Because that is where the other parts of the lag come from.Same conditions, just different resolution.
I never said that the screen size change is free - of course it adds to the lag. But it isn't the only thing that adds to lag. Even your own numbers are proof of that - they show that the fps is capped at 60.
Let's assume that you have a computer that can run a game at 90 fps (which is a guessed number). The engine limits that to 60fps. Those additional 30 fps are the reserve.
The change in screen size to 1920x1080 might have caused you to loose 30 fps or so - it will NOT have caused you to lose 52 fps, the difference from the guessed 90 to 38. There is something else that caused lag, costing you another 20 fps or so that you never saw from 90 to 70, before the screen size reduction came in.
And if you would find and remove that other lag, the reduction from the assumed 90 would still let you have 50 fps instead of the 38 you detect at the moment.
Or as another proof: make a new project. No events, no plugins other than a single plugin to change screen size to 1920x1080. And then test the fps again - it WILL be higher than 38 fps despite the fact that you are now at the same screen size where your other game reports 38 fps. Because that other game has more than one cause for lag to get to 38.