few things I noticed in a quick look over it,
many of your background sprites have collisions enabled and there is no need for them to have
your zombie movement change triggers could be much smaller and still be as effective (I still think a trigger once action with variables as triggers would be better here)
many of your weapon sprites have upto eight points in their collision "boxes" most would be fine with 3
particles are a performance killer..maybe make a sprite to look same and use that instead...
This did run at 60 fps on chrome for me though...