TL;DR
1: When a multiplayer server has no one logged on, the game should remain paused until the first player finishes loading.2: Add optional consent to pause game while others log on.
Problem:
Factorio has a multiplayer feature that allows players to log into the game without interrupting everyone else who is currently in-game. It does this by quickly saving a local copy of the map and then running the simulation as fast as it can on the new player's machine until all the events are "caught up" and in sync with the server's live running copy. While this is happening, the new player has a "Catching Up" progress bar showing how far they are along in this process. This is the process of simulating everything which takes place during the time disparity between the point where the server saved the game and the point where the new player's computer received and loaded this file. The amount of time this takes depends on how many new events are being generated on the server vs how fast the new player's computer can process them.For very large multiplayer factories, it can become extremely slow because so many events are happening while the new player is trying to load and "catch up". In theory, if a factory is large enough, it would be impossible for the new player to process events faster than the server itself. In times like this, it is sometimes a good idea to have someone on the server pause or slow down the game so that the new player can catch up sooner.