Again, the 'alleged' drop in framerate, visually (not talking about the analysis on that video that got put up, talking about what my eye perceives) seems intentional, something to do with the way the way wants to present itself.
You can tell because when you pause the game and go into the 'book' menu, THAT part is running very very silky (the pages turn very smoothly and whatnot). But when you pause and return to the game, the game resumes its movie-like presentation. This is true even if there is only one enemy to fight and if Gabriel is just 'walking' about... and I can assume that it's the same if there's no one around to fight... not that I got a chance to do that very much, since the demo pretty much forces you to 'keep moving' in both of those stages. There was no point during the demo in which I got to just 'stop' and stand still.
Again, I'll draw the parallels of watching a film like Inception, and comparing it to watching a live Football Sports event. Two different types of presentations, with drastically different look&feel to them.