Your attack rate is controlled by physical presence of the knives. When thrown they physically go in that direction, until they hit something or they reach the end of their range, and then they physically go back to you. You cannot use them again until they come back and you have four of them. If you run towards them, they will come back to you sooner, which means you can throw them again faster.
I don't think I can do anything about the spawning and difficulty easily, sadly.
Thanks for playing and rating and feedback!