The resolution if 854x480, sprites are 32x32 with 4 frames and an animation for each direction they move, which is the standard four directional. They have some effects and behaviors but these are usually disabled most of the time.
We're using Node-webkit. Firefox usually runs much slower for me so I try to avoid using it. I thought it was my video card but I checked in the game and in that site you linked to Ashley and it said I was running under hardware acceleration.
The moving of the sprites themselves doesn't effect my framerate, it's just when they're animated that things take a turn for the worst.
I will make a demo CAPX and see if I can replicate the issue for my computer here in a bit. I have to leave right now so it might be a few hours.
Thanks for inquiry into my issue. :)