A tip while grinding.

While mob grinding put your view distance to tiny.

One reason is because it will reduce the amount of chunks the server has to load. Increasing the server performance.

Another reason why is in those unloaded chunks no mobs can spawn. Which means less mobs for the server in areas where they aren't needed, and more mobs to be concentrated in your area.

So while these changes seem insignificant, every drop in the bucket helps!

PS: You should also probably not be grinding when there are already over 5000 mobs present.