My server is freezing/lagging bad for up to a minute when zombies spawn. I am only spawning 25 of both murderer and scarecrow per kilometer on a 4500 size map. Is this normal?

This is likely because of a reasonable number of zombies spawning, probably a few hundred in a short amount of time. This isn't really something I have control over however since this is how Rust spawns the murderers and scarecrows. You could consider lowering the number of murderers and scarecrows, however I'm not sure how much of a difference this would make.