Minecraft Server Issues

I have been trying to create a hamachi Minecraft server for a couple of friends and me. Now the server is working but they have a problem when they connect.

They are just floating in the sky, and F3 says they are waiting for chunks to load. After a while, they are forcefully disconnected, so something must be wrong with internet.

I have been looking everywhere for a solution. I have done this in the past for mod packs and older versions of minecraft all the time, and I have never experienced this issue with Minecraft.

We have already made sure, double checked firewall permissions are in order, we are connected and can ping on hamachi server, so I just have no clue what the problem is.

Can low PCs connect to servers with no issues?

So I’ve just got an i9 9900k with 32 GB of ram. If I were to run a modded server, could my family connect to it with lower end PCs with little to no lag or issues?

Is there anything I can do to make them able to join issue free, obviously other than get them better computers?

How and what affects lag and FPS drops? I’ve read that’s the main issue with Minecraft. Can we use it with optifine? Should optfine be installed on server?