Why is Heroes of the Storm so laggy?

I've been playing Heroes of the Storm for a while now, but I've noticed that the game tends to lag quite a bit. It's really frustrating when I'm in the middle of a big team fight and suddenly everything slows down. I've tried all sorts of things to fix it, like adjusting my graphics settings and restarting my router, but nothing seems to work. Can you tell me why Heroes of the Storm is so laggy, and what I can do to fix it?