damn, downloaded it, and while it's incredible to view, I can't do anything with it. the blank canvas alone still has lag. It did heavily inspire me on how to use the pieces though. (and how not to use them, cause lag lol)
I don't recall having any framerate whatsoever on the canvas. Did you generate lighting on it? That may have affected it.
That's most likely the problem. Our foundry map in custom games does not have any dynamic lighting, so it shouldn't cause as much framerate as you are seeing.
This is the most accurate I've every seen a remake be. It's too bad that with such immense accuracy comes immense lag. As for others not experiencing drop while others do, I believe that the Slim consoles aren't affected as much.