minecraft-java-edition,minecraft-mods,minecraft-industrialcraft – Troubleshooting Luminators Not Working in Minecraft IndustrialCraft

This is probably a case of chunk corruption. There is way away to fix the chunk, but BE WARNED!! This shall delete your builds.

1. Go into MCEdit (IF you don't have MCEdit, download it and open your world)
2. Select said chunk
3. Delete it.
4. Restart your game.

If you just want to get rid of mobs, you can select a chunk within MCEdit and there is a "delete mobs" button somewhere or "delete entities," something among those lines.

Honestly I am not sure there is much we can help with. if you can load a number of people locally and see no issues but have the same number of people remotely and there is an issue then its clearly the network would appear to be the blame because the server doesn't care where the people are from for performance. It gets/sends the data over the network to wherever.

Secondly though, we more need to know your computer specs and performance and such. Look at your CPU and RAM usage when the system is running just fine as well as when it is reporetedly running slow and see if there is a noticeable difference. If your CPU is peaked and you are seeing Can't Keep Up messages in the server logs then the CPU is too weak to support things. This is where you will normally start to see block lag show up and often happens more so when terrain is generating (people are wandering) than if they are staying in one place. If your RAM is peaked then it means the people are loading too much and you may be getting into disk swapping which is way to slow to be good for a minecraft server. This could also give you the cant keep up messages due to that and give you some block lag (which if you don't know is mining a block and it taking a while to break or opening chests/doors and it taking a while to respond).

The Rain issue is client side however, that implies its their systems that can not handle what is going on well. Have them turn down/off their particle settings.

With out more information I really can not offer more suggestions or things to help tracking down.