To be honest I've seen many games and helped with many, so I really don't remember which one you are referring too.
To answer your question:
Physics behaviour is very CPU intensive, especially if you use many objects and use collision polygon for collision detection.
Are you using the cocoonJS physics engine?
Faking the gravity is possible, but I doubt it will help you improve your performance, it could though.
Maybe you should have another look at your game and decide if you really need physics or if working with bullet-behaviour with gravity would be enough with some tweaking.
I have once created a custom movement faking gravity for a game, because only the player object really needed gravity-like behaviour, this worked so it is possible. (In the end I had to pull a lot of rabbits out of my hat to prevent fps dropping, for it also had issues with collision detection.
If faking gravity is your only option I could have a look to see how to implement it in a way that would be beneficial, but I think dropping physics altogether and using other behaviours to fake the physics behaviour could be of more use.