3 to the client and 4 to the server with a total of 8gb is never going to work. You are leaving your system less than 1GB for all other processes. Simply not going to work.
Pre generating chunks can be down with various server side mods and plugins. I for one use Nucleus along SpongeForge to generate my chunks before players are allowed on.
Generally this is an issue pointing at the server being lagged to death. Increase RAM if you can. Add a world border. Pregen chunks if you can. Etc etc wtc