For what it's worth it runs brilliantly on Chrome.
Your problem might, and use the term jokingly, come from the fact that at any given time the game is handling 500+ objects.
This is pretty easy to cut down, firstly you can use a library layer to store all the objects your current layout isn't using.
The number of enemies could also be decreased, I was completely swarmed out of the gate and would never have got past the second group if I hadn't realised that touching your health bar clears the screen of enemies (A feature I imagine won't make it to the final build!)
Maybe trim the particle effects a little bit? When your triggering it for every attack and reaction and can imagine it adding up.
Overall though I really liked the game! Nice spin on the whole Abe Lincoln Vampire hunter deal!